Adding a source to a profile that already has some sources does not qualify for the Sourcerers Challenge.
Generally the only profiles that qualify for the Sourcerers Challenge have the heading of UNSOURCED in brackets.
If a profile does not have the UNSOURCED tag, but like others have said, if it has not had any sources for an extended time period, then it will be accepted for the Sourcerers Challenge.
EDIT: you should be able to find a list of unsourced profiles by going to the top of any wikitree page and clicking on FIND and going down to UNSOURCED PROFILES and click on that.
By clicking on any of those profiles, you can see what the unsourced templated looks like.
I just checked it out and there are only 12 profiles in that list! I know there are a lot of unsourced profiles, so I don't understand what the list problem is.