I once transferred United miles to British Airways without any loss using Diners Club (Diners was having a double transfer promo with BA then).
If you want to top off miles in a program it is better to use other partners - for example if you are checking into Hilton you can present one of several airline cards to get 500/1000 miles. Also you can transfer Clickrewards to one of several airline programs.
Also if you are transferring to Hilton, it may be better to leave the points there and use Hilton points for airline awards.