Yes, you can connect your old guitar speaker cabinet to your KPA (direct in case you own a powered KPA or with the use of a cheap - or expensive power amp).
You could even put the KPA in front (or into the loop) of your guitar amp.
You will miss most of the KPA fun.
Your guitar speaker will always sound like your guitar speaker - and the real speaker plays a big part of the game.
In case you did not try it before - use a large FRFR speaker system - and get to the next level.