Let me use an example: if I have a soup that is passed the bishul point and all the contents within the soup are officially cooked. Is there an issur of putting it in area where it will get hotter? I think it shouldn't be a problem because it's already yad soledet.
As far as the m'lacha of cooking goes, it's fine as far as I can tell. (See Shulchan Aruch 318:15, which discuss dry food, but the same should, I think, apply to hot liquid.) But there's also the ban on seeming like cooking, which bars us from putting things near a fire. (See Mishna B'rura there, :92 and :94.) But certainly don't take my word for it! Consult your rabbi.