UbaidUllah, please don't post duplicates; we're a small team and it makes it difficult to keep the forum organized. Keep to the existing thread
UntionSdu, there are many ways of accomplishing this modification, but some of them will cause lots of upgrade headaches because the number of modifications you'll have to make will be considerable. I would suggest allowing OJS to continue tracking two separate roles, but modifying the enrollment process so that only one of the two is shown and both are assigned when a user is given one.
You could accomplish this with a few template modifications to hide the extra options, and a modification to RoleDAO.inc.php to cause two roles to be inserted when one insert is requested.
I won't be able to guide you through this at a high level of detail, but can help with suggestions where you need them.
Public Knowledge Project Team