Integrating a q/a script

Hello There,

I am totally new to WHMCS. I have to accomplish two goals
1. Integrate a third party q/a script with WHMCS.
2. Force user to create a username when he tries to use the script for the first time.

After going through the documentation available for WHMCS, I figured out that for the second question I will have to create a hook which will make the client to create a username. However, as per documentation hooks can belong to any of the mentioned category and will be created on the actions supported for ex: Client->ClientAdd.

Could anyone suggest how should I proceed to address above problems?
Do I need to create a module/addon which will function as a wrapper for the third party script?
Will it be better to create a q/a module for WHMCS instead of using the third party script?

Help Appreciated.

Thanks