If you have United Premier Silver status or higher you can convert your United Miles to Marriott Points at a 1:1 ratio.
I do not know of a way to convert AA or Delta miles to Marriott Rewards Points.
Thanks for the info. Did not realize you could do this and had about 12,000 miles expire a couple of months ago since, like you, I seldom fly United. I did get a message that I can reinstate for $125 or about 100 points/dollar. Going to do some figuring to see if its worth the money to reinstate and then transfer to my Rewards account.
Since its a 1:1 ratio it would not be worth paying the fee to reinstate your miles to transfer them to Marriott unless you were planning to purchase that many Marriott points anyway. According to TPG United Miles are worth an estimated 1.5 cents each while Marriott points are worth an estimated 0.7 cents each.
United sometimes has one way award flights available for 12,500 miles. If you are able to utilize one of those award flights then it maybe worth the fee to reinstate those miles.
Unfortunately that will not help loftus with the airline miles. You have to have United Premier Silver Status or higher. The credit card would only give the holder Silver Status with Marriott. However Marriott Reward Members who are Platinum are eligible to receive United Premier Silver Status as a perk.
I forgot to mention that your Delta miles will be ok as Delta miles do not expire. You could always try donating a small amount of your miles to keep your United and American miles from expiring.