Hello Zorawar
Yes I know its a big work around but one more valid point here is you don't create group account very often. Check the volume of creation of group account in a year or month. Hardly one create group accounts unless its really required.
To start with, you can load all the group accounts in MDG (as FSI entity type first and then ACCOUNT). This will minimize user's effort. Loading group accounts and accounts in MDG is not a big task. 1 day is sufficient to load both as normally the group accounts are less compare to operating GL.
You can't do anything using data model as your process will become complex. You can write a code to copy to update the table - 009 for group accounts directly once you create account but again this is not the standard way.
Inform your users that this is the only way and hardly they will create group accounts. Yes its double efforts but then you don't have any choice as of now.
Thanks
Kiran