I am currently converting a very old, but working classic ASP site to ASP.Net.
It has a completely custom written user management system. Whilst it work
If the existing provider works (has the right fields for your data), use that to start. You can VERY easily replace that with a customer provider later (just a single config value change).
Beware there isn't an "out of the box" ASP.NET management interface for that, you'll need to roll your own or use a third party one.