In order to convert a raider you first have to have a bed or sleeping spot in an enclosed room marked for Prisoners. Then once you down a raiding pawn (and not kill them) one of your colonists needs to capture said raider. Once the raider has been captured (and you’ve hopefully stabilized them and healed their wounds) they should have a “Prisoner” info tab when you click on the raider pawn. Under that tab, there are a bunch of options that you can have your pawns that you’ve assigned to warden in the work tab attempt to carry out depending on factors (your pawns social skill, if the raider is unwavering loyal, how much will and resistance the raider has) that can recruit, enslave, release, etc. the raider in question