PEX kill rankup help

Hello, I use PEX (PermissionsEX) for my permissions on my kitpvp server and I was wondering if there was a way to automatically join a group once at certain amount of kills. For example, my default kit is peasant (0 kills), then my next kit is archer (50 kills). All the groups have been made i just need a way maybe using "options" to rankup the players.

Help is very much appreciated

(Sorry for my bad English, I'm french)