I am minded to think you should be ok. I've got done this myself but I know you can get a real surefire foregrip on the TM recoil g36k, which should be the same body as the g36c. I can't find any examples but I'd testify in court that people put on RS g36 foregrips onto ares g36s. As I understand it, WE use the same type of body as the Ares.
There are slight differences between g36 bodies, I have put a real sight rail on CA and JG g36s (which are clones of the og TM design) and it just about fits, but not 100% - some dremelling / filing is required to make the front bolt line up properly.