Securityadmin role permissions

Is it expected to have users with securityadmin role to be able to add and/or edit existing administrator users?

Yes, this is perfectly normal.

Securityadmin role is a requirement for Comon Criteria certification, and clearly states that users with this role should be able to:

• See all administrators defined on the unit.
• Add/delete Administrator, change/modify existing administrators no matter who initially created them.