Hhonors' website says you can do one of the following:
a. stay at a hotel within the Hilton Worldwide portfolio pursuant to the terms herein;
b. earn HHonors points through any third party source of HHonors point accumulation, including a Hilton co-branded credit card, pursuant to its terms;
c. purchase HHonors points; or,
d. donate points through The Hilton HHonors Giving Back Program.
C or D is probably easiest. You can buy points here:
http://hhonors1.hilton.com/en_US/hh/points/purchase.do