Just like Gurekam74_or_gurekam said you can use this and then add some sort of an killed mob detector that provides the player a spawn egg to place in the spawner if he kills a creeper or some mob. I know this is a bad answer to your question but this is exacly what I was looking for. So thanks Gurekam
Sorry for bringing this back to life but, use this as an example
/scoreboard objectives add Stone stat.mineBlock.minecraft.stone
run that in ur chat
that adds a scoreboard objective, then to testfor it.
that in a command block with a fast clock.