Why can't I add my help desk managers as resources to new queues?

This could mean that the help desk manager only has access to the Help Desk Manager role.

You need a data security policy (DSP) that grants access to all requests. To grant required access for your business needs, assign both the HR Help Desk Agent role and the HR Help Desk Manager role. This allows the DSP that grants access to all SRs in all queues.