If you dig deep enough into the background and find a khorne sorceror he's probably a genestealer magus, riding a zoat fallen to chaos.
It's your game and you can do what you want as long as it's fluff and not RAW atmo. Just don't expect many people to like your fluff due to the vast majority of established canon going against this.
And as for summoning daemons it does need ritual. And in khornes case this translates as murder, skull taking, combat and gladiator games ect. Just take a chaos lord with a mark of khorne and pretend that he's been up to mischief off the board. Just don't expect to drop prescience.
Sorry I understand you want to actually summon a bloodthirster midgame....why don't you put chains on your psykers if you all you want to do is summon daemons of khorne and have them as 'sacrifices' for possession.....actually chains might lead to slaanesh!
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