Some Poskim rule it is forbidden to do so due to Hatmana Shemosif Hevel. Other Poskim rule it is permitted to do so and that so is the custom to be lenient. If one punctures holes into the foil or plastic, or the food is not fully wrapped in them, or the plastic is very thin and is able to absorb the food in the pot, then it is allowed according to all, as the food in the pot enters into the wrapped food. Furthermore, if the food is not yet cooked, and its form of cooking is when it is submerged within the food then it is permitted according to all. Thus it is permitted according to all to place raw wrapped Kishkeh into Cholent from before Shabbos.
 However, if it is not on the fire then according to all it is allowed.
 Shabbos Kehalacha Vol. 2 page 37-39; Piskeiy Teshuvos 258:3l; To note that this question has already been hinted to in the Halacha in which Admur rules that one may pour water into the hot springs but may not submerge them within a container. Thus, in essence the question here is with regards to if we consider the wrapped food to mix and be one with the food that it is submerged in or not.
 Minchas Yitzchak 8:17; Kinyan Torah 4:24; Shevet Halevi 3:47; SSH”K 42:63; See Netivos Adam 1:8
 Rav SZ”A in Tikkunim for SSH”K ibid; Vayeishev Moshe 1:20; Nesivos Adam ibid; Maor Hashabbos 2:24; Imrei Avraham 15; Or Letziyon 2:17-13; Sheraga Hameir 4:63; 6:141; Az Nidbaru 6:78; Lehoros Nasan 7:12
 Shabbos Kehalacha ibid; Piskeiy Teshuvos ibid; Maor Hashabbos ibid
 Minchas Yitzchak ibid