I still want the flexibility of the form designer in the activity-designer though. So my app will be slightly replicating the activiti-explorer when it comes to form rendering.
Found a way to do this, set a variable in a ScriptTask, e.g "userGroupList=admin", right before the User Task, then in the User Task's form, just set the type to String, with the name = "userGroup", when the renderer see this special name, it will use a drop down list instead of a standard input field. This will work well with testing the workflow in the activiti-explorer during development as well!