External module integration

Hello ppl,
I am trying to experiment with asterisk with our telecom software.
I would like to know whether its possible to integrate an external module with asterisk to avail the basic PBX functionality?

Its just a thought to avail some features and have instant messaging between the softphones.

Just give me some pointers to start the quest. I spent about 3weeks to install asterisk and other PBX products based on asterisk, and try to understand, but I could NOT achieve it yet.

Thanks for any help.