Hi Christian,
Unfortunately dynamic forms (the feature to dynamically fill fields at runtime) is only working for single select fields. You are very welcome to place a request for that feature to be changed to all available field types in our idea place SAP NetWeaver Identity Management (SAP IDM): Home since this kind of requirement is not unique - I have been in same situation already.
For more details check help topic "Specifying attribute values for an identity store attribute".
Anyway, the only way to achieve that now is to do it in a workflow (using approval tasks). Means in a first step you select the user and next you get an item into your inbox where you can select the roles you want to copy from that user.
Regards
Norman