Well first off, yes and no.
1.) You can't make a building (doodad) that has both a small outside and a large inside that you can enter.
2.) But you can set up a trigger that moves a unit that enters a spot near the "outside" doodad, to a different area representing the "inside". There are even some wall models on this site that you could download and use for walls on the "inside" part. You will also need to set up an area on the "inside" that takes you back to the "outside" using a similar trigger to the one you used to get them "inside".