Import users - Multisite - e-newsletter

Hi. Please are you able to advise your recommended procedure for importing users into a multisite environment.

I want to ensure they are auto subscribed to e-newsletter on the main blog but there are several newsletter categories to assign and the members also have company names, addressees and telephone numbers.

There are approx 25,000 so manually assigning them is out of the question

Thanks in advance

Stef