From Membership rewards you can transfer to several hotel programs from which transfer to United is possible (Marriott Rewards and Holiday Inn Priority Club also come to mind). But the conversion from Membership rewards to all hotels is quite poor. It is not just Hilton where the conversion rate is bad, other hotel programs are just as bad - though a bit more apparent with Hilton because you can convert airline miles to Hilton, but not airline miles to say Starwood or Marriott.