It’s a pretty complex problem. The easiest solution will be to modify the animation in your favorite animation tool so that the hand is where it should be. This would be something like 3ds Max, Maya or, I believe, Blender. I’m not sure how you could export the existing animation to make modifications though.
Another option, though far more difficult, is to use Inverse Kinematics (IK). IK allows you to specify a location for a bone to try to get to, and the bone (the hand in this case) will move in an attempt to reach that position.
Here is the Epic documentation for them
There are also some examples in the Content Examples project you can get that show a setup of it.
EDIT: After looking into it, IK is actually fairly simple! Check out the Content Examples project from your Epic Games Launcher, and load the Animation level. Down at the end there is a specific example showing how to move a hand to fit a gun!