Redmine

I have a workflow where I use a screen to change the assignee before a specific step. We want only assignees from a specific usergroup to be valid, otherwise the transition cannot be done.

To be more specific, I create a assignee screen (pop-up window) when you change the JIRA status from Open to Backlog.
And I also want the assignee can only be chosen from a group, such as Project Managers.

But I cannot update the Project Permission, because the rest transitions can assign to anyone.
Only this status transition need to be restricted to Project Managers.