Hi tomba11 and welcome to Marriott Insiders. I believe this is only possible with United Miles and its only because of their special Rewards Plus partnership with United.
You can however convert Marriott Points to multiple airline miles.
tomba11, you can use AAdvantage miles to book hotels directly through AA.com, including Marriott properties, but I don't know of a way to convert airline miles to MR points, other than the United deal mentioned above. Delta also has a thing called SkyMiles Marketplace where you can use Delta FF points to book hotels. Neither of these programs seems like a very good deal, but I suppose if you don't have any other use for your FF points, it's not a bad use.
Good question, tomba11!! Though I am not positive on the answer, I'm going to seek out better clarity. I will definitely keep you posted.
And I second our previous Insiders... Welcome to the Community!! (even though you've made previous posts )
Cara and I heard back from our team and they have advised that as of August 30, 2013, United MileagePlus Premier Members can convert MileagePlus miles to Marriott Reward points. The conversion rate is one Marriott Reward point for each MileagePlus mile. A maximum of 50,000 miles may be converted in a calendar year. Members may request a conversion by calling United Airlines at 800-325-0041 or online at United.com. United is the only airline which converts their miles into Marriott points.
If you have any further questions, please do not hesitate to ask. Thanks!
All alliances come to an end, sometime, and no-one can accurately say when. However no announcement has been made so I think it's safe to assume the alliance will continue into 2016. It is worth remembering that this conversion option is an elites only, UA Silver and above only, so although the alliance continues that redemption option can simply be withdrawn at any time, though more typically withdrawal takes place at year end, as in the case of the elites only E135 Marriott $1000 cheques (for 135,000 MR Points) which used to be a redemption option until it was withdrawn without warning at year end a couple of years ago