You can put the pieces back together, but it will be a lot of work - I would suggest you get new parts (see Option B). If you decide to go this route, you will need to reinforce the inside with either more ABS or some sign plastic, then use ABS paste to fill in the gaps on the outside. Then sand and polish 'til it looks like it never happened.
Here's a tutorial on making ABS paste:
Option B is to contact whoever made your armor kit and inquire on how much some new forearms would cost. May be easier, if more expensive. Good luck either way trooper.
EDIT: I see you have an Anovos kit, they unfortunately do not sell replacement parts. ATA or WTF are a good match. Good luck!