Extensibility and integration into existing sites

Hi all,
Recently I made a site using Typolight CMS. The client requested an online SHOP too.
I choose OpenCart - Very clear code and really MVC - great work.

What I want to do is to share the users from the site in the Online store which is based on OpenCart.
I.e the users from the site will be users registered for the online shop too.
Does anyone did something similar?
I reviewed the code and I though it will may be possible to use directly the TL tl_member table by extending it with fields that are missing. The digest algorithm for the password used in TL is sha1, while in OC is md5... etc. Are there any other major things which I should be aware off.
For example if the two system share the same cookie this will be great - thus when a user logs in to the site he will be automatically logged into OC and vice versa.
Is there a way to extend the system and not touching the CORE? Yes I can remove "final" from final class HelperCustomer and inherit from it but are there some other way to avoid this hack?

In addition:
Is there some ongoing effort to integrate OC with some popular CMS so I can see how it is done.

Thank you very much in advance. Any hint will be highly appreciated.

Is there some ongoing effort to integrate OC with some popular CMS so I can see how it is done.

I am actually working on an opposite approach, creating a simple CMS based upon Opencart's MVC framework. We did something similar for the old 0.7.9 version and hope to port it to the 1.3.2 version.

Very good!!! If you decide to publish it under an OS licence I may be of some help. I will at least review it and share MHO . Is the code publicly available?

Unfortunately Now I have too much (specific business logic) code written already and it will be a waste to leave it.
Anyway, any hint about what I intend to do?