The roles and access permissions to microflows, nanoflows and pages in Mendix are defined during development. They cannot be modified at runtime.
If you need flexibility in your authorization model you can add functional roles and assign them as needed.
no and it would be a big security issue if it was,
what i would personally do is the following:
create "functional" User roles for each of the different distinct Functionalities you offer in your app and then build a page where admin users can assign those roles
e.g.
Function 1 - Access 1 = "AbstractUserRole1"
Function 1 - Access 2 = "AbstractUserRole2"
...
Function N - Access M = "AbstractUserRoleNM"
and show the selection with boolean helpers to select multiple components per User