You can upgrade to a full BES 5.0.1 for >$2000, including 5 Additional CALs (there are cheaper offers in the usual places).
You could downgrade to a BPS (not really a downgrade, rather, a new installation if I remember correct) and buy additional CALs.
However BPS it a little step back from SBE - no remote search, no html email, also, the CALs cost some money. I dont know for sure if the current CALs will work with BPS, but they will work when you buy the upgrade to a full BES.
BPS will however sync your mail, calendar etc. like the SBE
If you want to wait, because rumor is, there will be a new SBE/BPS based on V5 in the future, if you have a spare SBE License, install a second Server, and put the users above the Limit of the current server on that one - I have not tried this, dont know, if this is a supported/possible configuration.
Maybe, someone else does know better about this ?
See here about the rumor here:
http://www.blackberryforums.com/bes-...requested.html
Or, if you really only want to add one user, maybe try to put some other user who only needs email on BIS.