is it okay for a non catholic to kneel in church?
I would say, of course.
If he/she feels comfortable doing it, a non-Catholic may do everything a Catholic does in church except receive Holy Communion (i.e., eat the consecrated bread and wine.)
Can a non-Catholic kneel? SURE!! Cross youself at the holy water fonts, say the creed, kneel, sing, pray, worship, and do anything else you feel comfortable doing that doesn’t distract from the Mass. Just don’t receive the Eucharist (yet).