Granting full access on partition

I need to grant (almost) full power on a partition to external users, in order to delegate administration. Unfortunately, the role 'manager' doesn't allow to manage certificates, which is needed here. And it is not possible to grant multiple roles for a given partition to a user ("A user account can have only one user role for each administrative partition on the BIG-IP system", according to the documentation).

As a result, those users need at least two different user accounts, which is are currently provided as a two shared accounts. I'd rather switch to nominal accounts, provided I can fix the previous issue somehow. Is there any way I didn't found ?