full shell user vs mail user

OK, I have a DH account with three domains. The main domain has email users, and basic IMAP access works great.

I am now looking into enhancing spam filtering with SpamAssassin options or maybe even DSpam, but I guess I have hit a basic problem right away:

For any of this to happen, I need a full shell user. I understand how I can create one, but how can I create/upgrade/link/whatnot such that an existing mail user xyz@maindomain.com also becomes a shell user?

Alternatively, how do I create a shell user such that it ends up with a xyz@maindomain.com email address?

I'd be grateful if someone could explain how the two account types go -or don't go- together. Thanks

Reply to self (w/ help from DH support):

When adding a FTP/Shell user, no email option seems to be given. Once you submit the addition, however, you'll see a page that lets you configure the email address for this user.

Works a treat, it's just not very obvious. Wiki has been updated.