How do you merge a level into a another WAD?

My friend is making level 3 while I made 1 and 2. How would I take that level 3 and replace the level 3 I have with it in my WAD?

http://www.google.com/search?q=How+do+you+merge+a+level+into+a+another+WAD&btnG=Search+the+Web

Tried it already. Couldn't figure it out much. Besides, im using tnt.wad, not Doom2.wad.

In XWE, open the wad that contains the most data, the wad with two maps in your case and then go to File >> Merge. This will allow you to select a wad to merge into the wad you've opened. The place where it inserts it depends on the lump in the list you have highlighted.

or you could use deutex / wintex,extract all lumps from both wads and merge the directories together,then edit wadinfo.txt to include all maps and run deutex -make yourwad.wad or direct wintex to make a wad out of the folder containing the wad structure

You can save maps in to other wads using Doom Builder.

EDIT: I forgot to tell you its called "save map into..." in the File menu in case you don't find it.