Hello!
That kind of depends, what plastic is used for the figures. If it's this soft, rubbery plastic (polyethylene?), it will be hard to do anything with them at all (like many paints don't want to stick to that plastic). But in case of hard polystyrene, I'd cut and reposition the figurtes as necessary. In case of complicated surgeries you could use wire to position the joints temporarily and then putty them once the pose is done, but that's complicated. You might also want to check this web page:
www.vietnam.net.pl/M48malymoden.htm
where I describe howq I modified the tankers to fit the subject I had in mind.
Hope it helps, have a nice day
Paweł