Assigning Transfers to a Category Group in Budget

Does anyone know whether transfers can be assigned to one of the pre-existing category groups, e.g., Rental Expenses, within the budget? I know that transfers can be assigned to customized category groups, but it appears that they cannot be assigned to the set Q category groups. I want to assign payment of a rental property mortgage (the principal part that goes to the mortgage account) to the Rental Expenses category group. (Yes, I understand that the principal part of the payment is not an expense in the traditional sense, but I can otherwise assign that transfer to a customized category group, just not the Q category groups, which do not appear when assigning categories to groups because only the customized category groups appear in the drop down.) Again, this is just for assigning to a category group within a budget. Thanks.