Plugin to change the login system

Hello everyone. I have a task to do and i want to know if WordPress could be useful to it. I’m working in an e-commerce website, and i need to have multiple blogs for each product, where people could post reviews, critics etc. The problem is that these blogs would be restricted, only customers registered in the e-commerce would be able to post on the blogs. My doubt is about it, is the wordpress plugin architecture able to allow me to modify the login system? Does anyone have any example or hint so that i could start developing my plugin?

I’ll be waiting for answers.

Regards.