when a user is created a row is created in mp_Users table and the user is automatically added to the role "Authenticated Users" creating a row in mp_User roles table. So migrating form another system you would want to put data in those tables.
I don't know of a good way to migrate passwords from another system unless they are in clear text it is not likely that you will be able to migrate them so you would probably need to generate a random password and then the users will have to use password recovery to reset it (there is nothing built in to mojoPortal to globally reset user passwords so it would have to be done by the user either clicking the recover password link or if they enter the wrong password 3 times in a row it will auto redirect to password recovery). probably best if the site is configured for clear text passwords before migration, so you would have to generate random clear text passwords on import, then you could change to something else after migration.
use of site membership pro is not relevant to the problem of migrating data. smp doesn't care how the data was created, it only manages role memberships for existing user rows with a purchase history.
other than that I have no specific advice as I don't know anything about the data structure in joomla