No, you can't merge the SMF users with the phpBB users. The tables are completely different.
If you have only the forum files and not the database, there is no way you will be able to convert. If you do have a backup file of the database, then you can convert.
You should consider converting to phpBB3 instead of phpBB2. phpBB3 is a better message board than phpBB2. The convertor to phpBB3 works much better than the convertor to phpBB3. And support for phpBB2 will be dropping in a few months.
Have you hugged someone today?