I think he wants to put a 7 speed cassette on a 8/9 speed hub. It's hard to understand the OP though. If this is the case, then just add the spacer on the freehub body before the cassette. Nothing else needs to be done. You can't add spacers to the axle without changing the OLD. You could move the spacers around, but that is not needed.

If you want to swap the 8 speed freehub body for a 7 speed freehub body, so no spacer is needed, then you need to do as Retro Grouch says. But since you mentioned that you needed the spacer for the freehub body, I don't think this is the case.