You don't. SlumpED is exactly like XWE, a lump editor, and other stuff, but more technical. To put levels in a single wad, use doom builder, select the level you want to put in your level pack e.g.
Levelpack.wad only has 1 level.
If you want more than 1 level in the wad, open the level wad you want to put in the levelpack, then go to save map into, then click levelpack.wad, then it should add the level into the wad, resulting in this.
Levelpack has 2 levels
The MAINCFG can be done in SlumpED or XWE, but I highly suggest XWE.