The attached cofiguration contains a full code for creating DNN users from a custom "customer" table. You can use it a start point, and modify according to your table structure
Here are some possible errors you may encounter during importing process, so that you can correlate it to the OWS debugging information.
'USERID > 0 - USERID
'USERID < 0 - FAILURE
'USERID = -1 - UNIDENTIFIED ERROR
'USERID = -2 - Duplicate UserName / UserKey / Already Exists / Already Registered
'USERID = -3 - USER REJECTED
'USERID = -4 - INVALID USERNAME
'USERID = -5 - UNEXPECTED / CAUGHT ERROR
'USERID = -6 - BAD/INVALID PASSWORD
'USERID = -7 - INVALID ANSWER/QUESTION
ImportUsers.txt