Membership site

I am having difficulty trying to solve a problem that I have.

My website is a mixture between wordpress and opencart. The reason I have opencart is because my shopping cart is very complex and was easier to implement on that platform. However, I want to use wordpress because I like the membership plugins. So here is my problem.

I want to set up a membership site. However, I can’t protect the pages that use opencart. Is there anyway to make a splash page, with sign up and login, and then forward it to the main website. Then, I would hide the URL extensions so individuals could not bypass the requirement for a paid login. Any help and or recommendations would be appreciated.