It's possible if you have root access, but other than that, no.
Best thing you can do, if you use zedge or another app to download ones you actually like, find the folder they're stored in and rename them to start with the letter A (numbers might work as well), since they display in alphabetical order.
For example, I use the "facebook_pop" ringtone for Facebook notifications. If I really wanted it higher on the list, renaming it to "a_facebook_pop" should work. Not sure that the _ are necessary.