BTW, not sure if anyone needs this, but I converted the entire Maxscript on the Mixamo thread to Maya, including creation of the root bone, twist and IK bones:
rename "mixamorig:Hips" "pelvis";
rename "mixamorig:Spine" "spine_01";
rename "mixamorig:Spine1" "spine_02";
rename "mixamorig:Spine2" "spine_03";
rename "mixamorig:Neck" "neck_01";
rename "mixamorig:Head" "head";
rename "mixamorig:HeadTop_End" "head_end";
rename "mixamorig:LeftShoulder" "clavicle_l";
rename "mixamorig:LeftArm" "upperarm_l";
rename "mixamorig:LeftForeArm" "lowerarm_l";
rename "mixamorig:LeftHand" "hand_l";
rename "mixamorig:LeftHandThumb1" "thumb_01_l";
rename "mixamorig:LeftHandThumb2" "thumb_02_l";
rename "mixamorig:LeftHandThumb3" "thumb_03_l";
rename "mixamorig:LeftHandThumb4" "thumb_04_l";
rename "mixamorig:LeftHandIndex1" "index_01_l";
rename "mixamorig:LeftHandIndex2" "index_02_l";
rename "mixamorig:LeftHandIndex3" "index_03_l";
rename "mixamorig:LeftHandIndex4" "index_04_l";
rename "mixamorig:LeftHandMiddle1" "middle_01_l";
rename "mixamorig:LeftHandMiddle2" "middle_02_l";
rename "mixamorig:LeftHandMiddle3" "middle_03_l";
rename "mixamorig:LeftHandMiddle4" "middle_04_l";
rename "mixamorig:LeftHandRing1" "ring_01_l";
rename "mixamorig:LeftHandRing2" "ring_02_l";
rename "mixamorig:LeftHandRing3" "ring_03_l";
rename "mixamorig:LeftHandRing4" "ring_04_l";
rename "mixamorig:LeftHandPinky1" "pinky_01_l";
rename "mixamorig:LeftHandPinky2" "pinky_02_l";
rename "mixamorig:LeftHandPinky3" "pinky_03_l";
rename "mixamorig:LeftHandPinky4" "pinky_04_l";
rename "mixamorig:RightShoulder" "clavicle_r";
rename "mixamorig:RightArm" "upperarm_r";
rename "mixamorig:RightForeArm" "lowerarm_r";
rename "mixamorig:RightHand" "hand_r";
rename "mixamorig:RightHandThumb1" "thumb_01_r";
rename "mixamorig:RightHandThumb2" "thumb_02_r";
rename "mixamorig:RightHandThumb3" "thumb_03_r";
rename "mixamorig:RightHandThumb4" "thumb_04_r";
rename "mixamorig:RightHandIndex1" "index_01_r";
rename "mixamorig:RightHandIndex2" "index_02_r";
rename "mixamorig:RightHandIndex3" "index_03_r";
rename "mixamorig:RightHandIndex4" "index_04_r";
rename "mixamorig:RightHandMiddle1" "middle_01_r";
rename "mixamorig:RightHandMiddle2" "middle_02_r";
rename "mixamorig:RightHandMiddle3" "middle_03_r";
rename "mixamorig:RightHandMiddle4" "middle_04_r";
rename "mixamorig:RightHandRing1" "ring_01_r";
rename "mixamorig:RightHandRing2" "ring_02_r";
rename "mixamorig:RightHandRing3" "ring_03_r";
rename "mixamorig:RightHandRing4" "ring_04_r";
rename "mixamorig:RightHandPinky1" "pinky_01_r";
rename "mixamorig:RightHandPinky2" "pinky_02_r";
rename "mixamorig:RightHandPinky3" "pinky_03_r";
rename "mixamorig:RightHandPinky4" "pinky_04_r";
rename "mixamorig:LeftUpLeg" "thigh_l";
rename "mixamorig:LeftLeg" "calf_l";
rename "mixamorig:LeftFoot" "foot_l";
rename "mixamorig:LeftToeBase" "ball_l";
rename "mixamorig:LeftToe_End" "toe_end_l";
rename "mixamorig:RightUpLeg" "thigh_r";
rename "mixamorig:RightLeg" "calf_r";
rename "mixamorig:RightFoot" "foot_r";
rename "mixamorig:RightToeBase" "ball_r";
rename "mixamorig:RightToe_End" "toe_end_r";
select -cl;
joint -p 0 0 0 -n "root";
connectJoint -pm root pelvis;
select -cl;
vector $lowerarm_l_pos=`xform -q -ws -t lowerarm_l`;
joint -p ($lowerarm_l_pos.x) ($lowerarm_l_pos.y) ($lowerarm_l_pos.z) -n "lowerarm_twist_01_l";
connectJoint -pm lowerarm_twist_01_l lowerarm_l;
select -cl;
vector $upperarm_l_pos=`xform -q -ws -t upperarm_l`;
joint -p ($upperarm_l_pos.x) ($upperarm_l_pos.y) ($upperarm_l_pos.z) -n "upperarm_twist_01_l";
connectJoint -pm upperarm_twist_01_l upperarm_l;
select -cl;
vector $lowerarm_r_pos=`xform -q -ws -t lowerarm_r`;
joint -p ($lowerarm_r_pos.x) ($lowerarm_r_pos.y) ($lowerarm_r_pos.z) -n "lowerarm_twist_01_r";
connectJoint -pm lowerarm_twist_01_r lowerarm_r;
select -cl;
vector $upperarm_r_pos=`xform -q -ws -t upperarm_r`;
joint -p ($upperarm_r_pos.x) ($upperarm_r_pos.y) ($upperarm_r_pos.z) -n "upperarm_twist_01_r";
connectJoint -pm upperarm_twist_01_r upperarm_r;
select -cl;
vector $calf_l_pos=`xform -q -ws -t calf_l`;
joint -p ($calf_l_pos.x) ($calf_l_pos.y) ($calf_l_pos.z) -n "calf_twist_01_l";
connectJoint -pm calf_twist_01_l calf_l;
select -cl;
vector $calf_r_pos=`xform -q -ws -t calf_r`;
joint -p ($calf_r_pos.x) ($calf_r_pos.y) ($calf_r_pos.z) -n "calf_twist_01_r";
connectJoint -pm calf_twist_01_r calf_r;
select -cl;
vector $thigh_l_pos=`xform -q -ws -t thigh_l`;
joint -p ($thigh_l_pos.x) ($thigh_l_pos.y) ($thigh_l_pos.z) -n "thigh_twist_01_l";
connectJoint -pm thigh_twist_01_l thigh_l;
select -cl;
vector $thigh_r_pos=`xform -q -ws -t thigh_r`;
joint -p ($thigh_r_pos.x) ($thigh_r_pos.y) ($thigh_r_pos.z) -n "thigh_twist_01_r";
connectJoint -pm thigh_twist_01_r thigh_r;
select -cl;
joint -p 0 0 0 -n "ik_foot_root";
connectJoint -pm ik_foot_root root;
select -cl;
joint -p 0 0 0 -n "ik_foot_l";
connectJoint -pm ik_foot_l ik_foot_root;
select -cl;
joint -p 0 0 0 -n "ik_foot_r";
connectJoint -pm ik_foot_r ik_foot_root;
select -cl;
joint -p 0 0 0 -n "ik_hand_root";
connectJoint -pm ik_hand_root root;
select -cl;
joint -p 0 0 0 -n "ik_hand_gun";
connectJoint -pm ik_hand_gun ik_hand_root;
select -cl;
joint -p 0 0 0 -n "ik_hand_l";
connectJoint -pm ik_hand_l ik_hand_gun;
select -cl;
joint -p 0 0 0 -n "ik_hand_r";
connectJoint -pm ik_hand_r ik_hand_gun;
Select your skeleton and paste into the Mel command line.