I made an experimental 8 string many years ago, around '67 I think, with one string above the standard 6, and one below. The top string was the problem, there weren't many gauges available then and I broke many banjo 5th strings before giving up and making it a 9 strng with the top three doubled. It was......er.....interesting to play, but didn't really do a lot for me. I guess if I did it again I would add two bass strings below the standard 6. I much preferred the 9 which was a lot easier to play than a 12 string, but gave a similar effect. However, I have more than enough trouble with 6 strings to want any more now.................
Good luck though!