SPG are taken indirectly. Please convert your SPG to Marriott Rewards, and then to United Mileageplus and make order for United MileagePlus at http://milesp2p.com/united , which is very popular.

To transfer SPG to Marriott

1. If you don't have existing Marriott account, please apply for one at marriott.com.

2. Link your SPG and Marriott account at http://members.marriott.com/link-accounts/ 

3. Login your SPG account and click “Transfer Starpoints to Rewards Points.” Follow screen instruction to complete the transfer. Marriott Rewards should arrive instantly.

To transfer Marriott to United MileagePlus

1. If you don't have existing Untied MilesagePlus account, please apply for one at United.com.

2. The link to transfer is http://www.marriott.com/rewards/usepoints/morerew.mi . Transfer ratio is available at the link as well. The higher quantity, the better ratio. 

3. Before transfer, please check Popularity Info at  http://milesp2p.com/united to make sure that your final quantity of United miles is a popular one, so that your order is processed sooner. Ad-hoc quantity that is not popular may cause waste or miles, and/or extremely long waiting for your order to be processed.

4. After transfer is done, United miles arrive at target account is 2-5 days.

   
   

Note:

The price of SPG is calculated at relatively optimized scenario:  32k SPG can be transferred to 96k Marriott, which can be converted to 40k United miles which are priced at $540, making effective rate of SPG $16.8/k.  Your price may vary depending on quantity you have.

You may use other sources to get United MileagePlus as well.  For example transfer from Chase Ultimate Rewards,  or apply for credit card that comes with 50,000 or more United Miles.