Hi,
Probably many of you already know this as it is so 2015, but it might be patched with 0.63 and new animations, so I guess everyone can abuse it now as they wish.
It is relatively easy, but you will need a zombie, and it is possible that it will hit you, therefore be sure that you have rags or a bandage just in case.
I've managed to get into every building that contains an empty part, i.e. piano house, prison building or red house.
What you do is, get a zombie and make sure it runs fast but is not to close to you, to do that dodge the zombie and change your direction. You run at a corner in front of the empty part of the building, see from pictures:
When you're in the corner, keep on running and as the zombie clashes with your hit box, it should push you into the wall. Sometimes it is delayed, crouching or vaulting also helps. Notice that the zombie will be hitting you if you fail, it will stop when you're inside even if it doesn't seem as if you are. If it failed and the zombie keeps hitting you, repeat.
Avoid doing this with large amounts of zombies, as you can very easily die that way. When you're inside, you can't shoot or be shot through the walls, but you can easily walk out of them into the building when crouched, then you can shoot those inside. Walking seems to be easier when your back goes through a wall first. To leave completely, just go up some stairs or go to an exterior wall.
You can see it from a video by this guy, however ignore the method he is using as it is already patched. Behavior inside the walls isn't, so use his guidance.
I hope you enjoy and look forward to 0.63 as much as me