I've not used phpBB, but I'm assuming it uses standard md5 password hashing. If that is the case, you need to merge together your two user tables. Look in your mysql database (FlashMyAdmin) . wp_users is how WP users login. Find the phpBB users table and see how similar they are. Assuming they have more or less the same information, you need to adjust your login scripts so instead of looking in phpBB_users, it looks in wp_users, or vice versa. If there are mutually exclusive fields you need to use, you should be able to add them pretty easily. Click the wp_users table, and hit Insert New Field. Fill in the necessary data, and you should be good.
Hopefully you don't have too much traffic already. It's a bit more work to migrate all the old data over (Hint: click Data Maintenance in FlashMySql and play with the Import/Export SQL queries).