Multi-DB Support: Best way to import many members and groups to wp multisite

Hi. We have a multisite installation - http://www.SA-People.com - where we feature news posts and offer our members free blogs.

We would like to start offering them more interactive features - so I think we'll need to install Buddypress.

We have about 800,000 members from our previous non-wordpress site. We would like to:

a) import the 800,000 members to the wp multisite so that they can login without needing to create new users. What is the best way to do this?

b) set up the same groups on the wp multisite/buddypress as we had on the non-wordpress site. The most important is we have hundreds (maybe thousands) of schools, universities and subgroups (ie. primary, secondary schools etc.). Do I need to manually create all of these in Buddypress? Or is there an easier way?

The members would then need to re-register at their groups but at least if the members are able to login easily (a), and all the previous groups are there (b)...then it's an easier process for them.

Thanks.