Open a clean version of your map in HMT.
Do a batch extract (tools>extract>batchextract), into a new folder, name it Shef. Cause thats what cool kids do.
Open any map with the wraith in HMT
Go to projectiles.
Click the recursively button next to save, and click save, select the "Shef" folder as where to extract, it'll do the work with saving the right files for you.
Now copy paste the clean version into the shef folder, and select tools>Rebuild Map
Select the map inside the shef folder as the original map.
Select yes to auto place stuff
Under new tags, right click and hit add, and browse to the Shef folder, go under vehicles, wraith, and select wraith mortar.proj.meta.
Now click build map, when it asks about the new Vertices, hit no, same to indices, but YES to modified offset.
Now rename it to "(mapname).map", open your rebuilt map in HHT, and find any weapon you want. Find where it says "proj" somewhere, and swap whatever is there with the wraith mortar.
Tadah! Your first rebuild.
If you need more help feel free to PM me.