I don't know what "merging users table" really means for you.
If you remain keeping two users tables, one for your application, the other for JForum, then SSO would do the trick.
You can use your application user's username to login JForum by implementing JForum SSO.
See this post
http://jforum.andowson.com/posts/list/35.page as a reference.
If you just want to keep one users table for both your application and JForum, then you can keep jforum_users table
and add new columns into jforum_users for your applications.