What you can do is to create a trigger and a dynaform to edit and/or create users based in these two ProcessMaker functions:
PMFCreateUser()
http://wiki.processmaker.com/index.php/ ... User.28.29
PMFUpdateUser()
http://wiki.processmaker.com/index.php/ ... User.28.29
You can use a couple of dropdowns, one to select the role and other to show the user list according to the value of the previous dropdown, in this case admin role must not be in the first dropdown (to avoid modifying the admin account). Once the user's permission has been selected and modified you can hide the 'PROCESSMAKER_ADMIN' role to unable the user to be upgraded with an admin role. In case the manager user needs to create user's account it is necessary to validate the information to not let him to create admin users or assign any role with the PM_USERS permission.
The PM_USERS permission will only be assigned to the admin user and the manager user will only manage the users through the dynaform created, so that the manager user will not be able to upgrade his role or modify admin account.
Dependent fields might be necessary:
http://wiki.processmaker.com/index.php/Dependent_Fields
I hope it helps you to achieve your requirements.