hacking user table in db

Hi,

I am looking to use WP for a club membership site that will be ported from an old PosstNuke site.

My problem is the paid membership system they have now has some taters that I can not find in any other.

The way it was done on the Post Nuke site was they added extra fields to the user data base table that the membership system used.
Specifically:
* a member ID (not always unique family’s have the same ID)
* Expiration Date
* Admin notes
New members were also created by admins via custom form that did a direct add to the db (with site functions).

This with all the existing user data allows PayPal’s IPN to do the rest with a lot of custom back PHP admin files stuck in the PostNuke permission system.

So my question is can I / should I do it with WP?
I know building a plugin is better, but would doing this cause any big problems with updates or?

Thanks