Like ninthtale said, it might be better to make animations outside of Unreal, (like in Maya or Blender,) Especially if you want the mannequins movement animation to change. As I’m pretty sure Unreal’s animation editor (Persona) does not have many advanced tools. But I’ll go ahead and show you how to attach the gun to the mannequin, and offset the arm positions. Just remember: Since we’re not changing the movement of the animation, it will look good with small handheld items, (like a handgun,) but it may or may not look silly with larger items. But don’t worry, you could add some separate animations for large items (like long arms) later.
For attaching the gun: (I have attached a lamp in the image below.)
Import a gun mesh into Unreal. Now go to your third person BP and click on “+Add Component” and choose whatever your gun is, (static mesh?) and drag it below the mannequin mesh to make it a child. Now before you adjust the position, click on “parent socket” in the details panel, and choose the hand. Now the gun should be moving around in sync with the mannequin. Now use the transform tools in the top right of the viewport to disable grid snapping and position it as correctly as you can. If you click on the mannequin mesh, you can check “pause anims” in the details panel which might help.
Now for offsetting the arms:
Go to the animation asset tool for your mesh, double click on the animation you want to edit, click on a body part and use the transform tools to make adjustments, then click on “key” in the header to apply them, now save it, and the animation should be updated. 
