Sony Rewards program is now accepting trophies in exchange for credits according to the latest update. Members of the Sony Rewards program can now have their trophies exchanged in exchange for points which can then be converted to PSN currency according to GameInformer who first got hold of this information.
The exchange system will give members 100 points for 100 silver trophies, 250 points for 25 gold trophies, and 1000 points for 10 platinum trophies. 1000 points can then be converted into $10 worth of PSN currency. This offer, however, is only available in the United States and for those who just became members of the program, your previous trophies won't count, so you must grind for more trophies if you're a new member. Hopefully, Sony will bring this type of rewards system globally in the future.