There's 3 options for doing this, although one of them is "Temporarily Suspended" due to the United/Continental merger.
For the past few years United has offered a "Miles and Money" option when booking a redemption flight. This allowed you to partially pay for a flight with miles, and partially pay for it with "money". There still had to be award availability for the flight(s) you are after, and generally the amount of the money you had to pay made it only a good idea if you didn't have sufficient miles to pay for the entire trip. This option is currently temporarily suspended due to the United/Continental merger, and they have not stated when it will return.
Option 2 is to buy miles. This costs $35 per 1,000 miles plus a 7.5% excise tax, so it's really only a good option if you are only a few thousand miles short of what you need to purchase an award.
The final option is to buy 2 one-way tickets - one with miles and one with cash. United has allowed one-way mileage bookings for a few years, and there's no real disadvantage to booking one-way over return other the the fact you are not able to have a stop-over on one-way redemption booking where you sometimes are for a return. This option doesn't normally work out to be cost effective for International trips, as one-way international can cost the same as or more than a return, but for domestic travel it's a great option - especially if redemption seats are only available on one leg of your trip.