You can use a spacer at the end of the coil, not between. Coil spring spacers are sometimes used for a small or leveling type truck suspension lift. If one isn't commercially you can have some fun making one yourself from a piece (two pieces, really, one for each side) of suitable hard rubber or, if you're only looking for a small lift as I suspect, maybe some thick sheet steel.
Keep in mind that while adding spacers can correct a ride height loss, it won't correct for loss of spring rate. If it feels either mushy or harsh now, it will feel the same way with spacers installed. If you actually want springs that feel the way they should, replacement springs would be the most economical way to go, much cheaper than cryotreating and you know it will work.
