Yes, provided you don't exceed the total number of maps allowable [it's considerable anyway] you can place multiple '<any_name>.img' maps into a root folder named 'Map' on either the unit or microSD card. It's much safer to have unlocked 'unofficial' maps on a card, if one causes a problem simply removing the card lets the unit operate normally.

Best just buy yourself a microSD, even branded cost less than a dollar per GB.