I have a Shabbos hot plate without a knob that I use to heat up food that is dry. On Friday night before Shabbos I place cooked chicken and Kuggle on the hot plate in an aluminum disposable pan. Because the bottom of the chicken sometimes dries out and starts to burn, I add hot water from the urn to the pan. Am I allowed to do the same thing on Shabbos morning, add hot water from the Urn to the pan?

ב"ה Shalom and Moadim Lesimcha The halacha in this case is very clearly explained in Rav Eliezer Melamed's book Peninei Halacha on the laws of Shabbat, so I will copy what he has written with minor changes and a few additions for clarity. His sefer is fully available online. I will just first say that it is not clear to me what you mean by Friday night. If it is before Shabbat there is no question at all that you do on Friday is fine. However, if you are referring to when it is already Shabbat, then there is no difference between Friday night and Shabbat morning in regard to the laws of adding water on Shabbat. In general , there is another solution for warming the food without having it get dried out without adding water, by simply not placing the aluminum pan directly on the hotplate, but by keeping it elevated above the hotplate by some metal grate or the like , this way the foods gets hot enough but doesn't dry out . Peninei Halakha: Chapter 10: 13 If the water in the cholent pot evaporates [or any other dry food] and one is concerned that the cholent will burn, one may not add cold water to the pot, because the water will become cooked. But if there is an urn on the hotplate , one may pour hot water from it into the pot. If the urn has a faucet designed to let the water out, one may remove the cholent pot from the hotplate and pour the hot water into it by means of this faucet. If the pot is designated for meat and the urn is designated for pareve, one should first uncover the pot for approximately ten seconds, so that the pot’s steam will dissipate. One may then move the pot under the faucet, since only minimal steam will be released. If it is difficult to pour directly from the urn into the pot, one may pour the hot water into a cup and then pour it from there into the pot. This is because as long as the water is "yad soledet bo, " according to the vast majority of poskim the prohibition of Bishul= cooking on Shabbat does not apply. This is the practice of the Ashkenazic, Yemenite, and North African communities (Mishna Brura 253:84; Yalkut Shemesh §88; Shemirat Shabbat Kehilchata 1:17). In regard to the custom of Sephardim: Others maintain that one may not add hot water to a pot on the hotplate , because the primary issue is not how hot the water is, but the status of the water. While the water is still in the urn, it is considered a kli rishon. When one pours out the water, its status changes from a kli rishon (which has the ability to cook food) to irui = pouring from a kli rishon (which does not have the ability to cook). Then when the water reaches the pot, it is recooked, and can once again be considered a kli rishon. There are some Sephardim who follow this opinion (Yeĥaveh Da’at 4:22). If Sephardim wish to be lenient in accordance with the opinion of most poskim, they have an opinion to rely upon (Or Le-Tziyon 2:17:8; see Menuĥat Ahava 1:3:15). All the best shabbat Shalom and Chag Same'ach.