Several airlines provide transfers to Hilton HHonors. Simplest summary is the one on Hilton's site...
http://hhonors1.hilton.com/en_US/hh/...esforpoints.do
Be aware than some of these programs track the source of the miles and will not allow miles created by transferring from another program to be laundered. You cannot, for example, redeem Membership Rewards for Mexicana or Hawaiian miles and then redeem those for HHonors points.
Likewise, Amtrak Guest Reward only permits outbound transfers (to any other program) for AGR Elite members -or- for members who hold the AGR credit card -and- have used that card to spend at least $200 with Amtrak during the year. In other words, you must be an active Amtrak customer.