Spacing on 8 speed vs 9 speed cogs is quite different. Very strong chance that unless you use friction shifting, a 9 speed index shifter won't properly shift a 8 speed hub with an extra cog.

Also how would you add the extra cog? the 8-9-10 speed cassettes are the same physical depth, the only real difference is the thickness in the gears and the thinner spacing.

If you put another cog on an 8 speed cassette, it would be too big for the locking ring to tighten down on the hub.