If this is an upgrade process that we are talking about, the best way is to use the old database and perform an upgrade procedure in order to make your db up to date with the latest version. You can find SQL upgrade scripts in the /MonoX/Installer/Sql/UpgradeScripts and then just run the scripts by version from your old version to the latest.
If you don't want to go this way then you can try importing you users with the SQL script (just select users from the old db and insert them into new one) or use some tools that can help you migrate users.
The following tables are involved in the transaction:
Note: If you copy your users from the old db you will not copy the content they have produced e.g. wall notes, blogs etc. so keep in that mind.