Press F1, and warp to the 2nd map. Create the 2nd map, as if it were the inside of the shop. Get the coordinates you want to warp to. Warp back to outside the building. Click on "Attributes" on the map editor. Then click warp. Change the map number to the one you want to warp to, and place the "warp" symbol on the map. Just like that, you are done.