I mean, you can. You just need a credit card to pay for taxes/fees. Any credit card will do.

But, how do you acquire an United credit card, earn Kirby bux and redeem through Lifemiles?

I am not a fan of airline co-branded credit cards, as they'd limit redemption options.

Also, economy cabin should be in play as well. Have you look at the cash price on some O/D routes? And, with United's dynamic pricing, you would be likely paying around 1cpm in award redemption anyway. Like I said, having other options are better than no having those options.