You mean adding units to the MPQ? I dunno if you can add them, but it might be possible to replace/edit the default units, if you can manage to find where the data is stored.
Also, for expansion-only units (Spell Breaker, Shadow Hunter, etc.) you will have to edit the expansion MPQ as well (War3x.mpq).
If you do plan to do this I think MPQDraft can do it, but don't expect it to be as simple as editing units in the World Editor.