You should be able to do it yourself at midwestairlines.com. NWA.com should show you the yx confirmation #. If they don't, then just call YX. Either way, you'll be able to get seats on YX now.

I've got the opposite situation. Booking a mixed nwa/yx ticket is cheaper on yx.com (nw op, yx codeshare)...>90 days out so I can get my yx seats but have to wait for the nwa seats.

I was surprised that with an nwa flight on yx.com, you can still do the 24h hold, guaranteeing the fare. It also applied my 5% yx mastercard discount to the nwa fare!