You could use “Layered blend per bone” to keep the animation of your locomotion from your lower body but add some fire anim montage to your upper body.
In Anim Graph:
Drag a wire from your state machine and create a “New Save cached pose…” if you don’t have one already.
Add a “Layered blend per bone”:
Use cached pose ‘Your statemachine cache name’ → Base Pose
Use cached pose ‘Your statemachine cache name’ → Slotname of Animmontage → Blend Poses 0
Click on “Layered blend per bone”, click on Config, Layer Setup and add a new element at “Branch Filters”. At Bone Name choose some bone of your characters spine. Enable “Mesh Space Rotation Blend”.
Pull a wire from the outgoing side of your Layered blend per bone node and connect it to your Final Animation Pose (or to your AimOffset first if you have one).
If the character does not stop movement afterwards but even does not show your anim montage - or just a little bit then check “Blend in Time” in your Anim montage (it should not be too high in case of fire anim).