You can do that now by posting a comment to the thread. I realize that's not exactly what you mean, but (a) it's close, (b) we're biased in favor of reusing existing features wherever possible, and (c) I'm not convinced that the submitter's comment should be privileged over other users', other than by being first to appear. What chippy wrote was fine, but that would not always be the case.
I don't think the submitter's comment needs to be privileged. But it would be nice if both text and url are entered then the system just magically turns that text into a comment from the submitter. It would just save the submitter a step. I'll refrain from the typical dev comment about how it should be easy to do in the code. It probably isn't that easy. But probably not that hard either. :)
