Hi Dave,
Assuming you have done a default installation of PSW, including MySQL:
The MySQL database can be fully accessed (from local system only by default) and modified.
Ofcourse you will have to identify the fields and match them to your existing database, but that is a one-time job. After that, you can realise even a scheduled synchronisation of your user base if you would want to, or create a registration form on your website that will create new users.
We offer with PrintShop Web the building blocks based on open standards, you can modify the database contents, so you can use any means that can write to the database you can imagine.
A great (free) tool to view the PSW database contents can be downloaded here:
http://www.mysql.org/downloads/gui-tools/5.0.html Kind regards,
Floris