Sure. Just follow the following steps:
1. Open World Editor and import
Dragoon's Model. I presume you know how to do it.
2. Open the Object Editor.
3. This time you will use the Human TownHall/Keep/Castle unit as a base.
4. So, go to the Town Hall unit, and change the Art-Model to Dragoon's Goblin Hall.
5. Do the same with the Keep and Castle units.
6. Now, notice that at the section Art- Required Animation Names of the Keep unit it says "upgrade,first", and at the same section of the Castle unit it says "upgrade,second". This is because in fact the three units share one model which has three different sets of animations (one for the TownHall, one for the Keep etc.) and the section Required Animation Names specifies which set the model uses.
7. Now if you've done all the previous steps correctly, you have a fully functional 3-stage town hall. Now comes the part about the extra tier Dragoon added. Do make it show in-game instead of one of the other ones simply go to the Required Animation Names section again, and change the "upgrade,first" or "upgrade,second" to "upgrade,third". This way it will show you the highest level of the town hall. But since you said you needed a 3tier goblin town hall instead of 4, i suggest doing steps 1-6 and you are good to go.
8. Just a tip: scale the model to 3 'cause its a bit small.