[pushkins]

Compaction of any fibre insulation always reduces it's R value.
Adding ply wood to a 6" ceiling joist to use as storage is not highly recommended but under some sircumstances can be done.
You must be aware that whatever you put up there in storage it's weight will have a direct effect on your ceiling.
If you want to store a Christmas tree and decorations then that would be okay however if you want it to store books for example then that wouldn't be okay.
If you still want to turn this space into storage then I'd suggest you buy 3/4" plywood for extra strength. As for the insulation problem under where you want to install the ply wood I'd remove the existing blown in insulation and use it to top up other areas in the attic and replace it with hi density insulation, that way you don't need to worry about compaction.

Make sure any added insulation is UNfaced.

Good Luck