The maps need to either A) have the portal placed in the level as a thing, or B) have it's exact location and angle defined in the level header. I assume the mod you're playing on doesn't have the Lap Portal thing because... Why would it, and even if the level header was left untouched by the mod, the location of the portal would probably be in some wall. Your only option would be to take the map out of the map pack's pk3, go into the map using zone builder and place the Pizza Portal thing in the desired spot.