Typing "Skywards To Marriott" into Google found the following information in less than 1 second.
You cannot convert Skywards miles to Marriott. However, you can redeem your miles for one night. However, the minimum number of miles is 15,000. See below for details.
http://www.skywards.com/partner_deta...=tcm:136-81480
You could buy some Skywards miles if you really wanted to.
Cheers,
Rick