Convert from other kits works. I wanted more Transuranic Arqubuses so badly I built 2 from scratch with various bitz and plastic pipes.
As for weapons such as the plasma calviers and ARc rifles. strapping tubes and such to standard plasma guns should suffice and the arc rifles design is such a way that I can imagine modifying a lasgun with sprue and other bitz could function adequately.
I also have about 5 skitarii themselves cobbled together from left over Tempestus body parts with heads from the Onager crew and left over heads from the other two squads of troops.
Its a pain but getting additional special weapons for skitarii can be done.