Step 1 might be to put it into Romajii, or whatever that term is, and just use it like that.
"Kai-yukkuri ni natte ageru", lit. "to become a pet yukkuri [for anon]", but bearing the nuance of the speaker doing a great favor for the listener, who is of a lower position on the societal/power hierarchy. Think of the situation when a bully gets bored with you and decides to stop messing with you for the day. "-Te ageru" (or it's much more informal equivalent "-te yaru", more likely to be used in the bully example) is the kind of grammatical structure that makes people feel the urge to punch you because of your high-and-mighty display that piss them off, if not prevented by a hierarchical barrier.
So a yukkuri saying "Kai-yukkuri ni natte ageru" would be purely out of contempt for humans, thinking that anon should be grateful that the high and noble yukkuri decided to leave its royal easiness to descend and live with the lowly human. And you're fine with stuffing all that explanation into the translation? I sure as hell am not.
@TosylChloride "Marisa will be so kind as to become Mister Human's pet yukkuri!" Since the implied rudeness is the key point of the sentence, make it explicit.
Masha will be sho kind ash cho bekwome missh human'sh pech yukkuchi!Maicha is shuitable for being a pet yukkuchi noje!Eajy!?Masha ish MashaMacha ish eajy!Look at Maicha'sh mishter hipAquatic Marisa scooping