Option to add permissions or role instead of change

It is possible at the moment add permissions or roles instead of change completely??

I want to bring the option to a certain roles that they can masquerade as a user but mantining his permissions or user role. It can't be some that is activated by default but it can be useful in certain situations.

Imagine that you are doing a Point of Sale (POS) transaction in a Drupal commerce website. You can masquerade as a user in order to do the order with the name of the user that is buying on the physical shop. But you have some permissions on line item fields that only administrators can use and are useful to do a good POS sell. It will be very useful that admins can keep his role while is masquerading as another user.

Thanks a lot!