You can give any user in Sage HR Online Services administrator access. For example you could add an employee record from Sage 50 Payroll, then edit their permissions to admin.

However, if you want to create an admin user who doesn't need to be linked to an employee record in Sage 50 Payroll, or that user isn't on the payroll, you can manually add a new user in Sage HR Online Services and then assign them admin access.