The conversion rates would be atrocious but you could do United to Marriott to SPG to Delta (or once the programs combine United to Marriott straight to Delta).
Limit is 50k a year at a 1:1 ratio so 50k UA to 50k MR to 16.6k SPG to 16.6k Delta (or wait until you have at least 20k SPG so you can transfer and get bonus 5k for 25k Delta). That all being said I would never do this.
This is part of the RewardsPlus thing UA/Marriott have. Details
https://secure.unitedmileageplus.com..._to_points.jsp