Q. Can you freeze hadassim still in their sealed plastic bags, that were not used, and use them next year?

A. Igros Moshe O. H. 1:185 maintains that an esrog that was frozen is not considered kavush and could in principle be used next year Sukkos. However, he writes that one has to be careful as it may have began to rotten inside, since it is common that frozen fruits spoil shortly after being unfrozen, and one should not use such an esrog.
Other Poskim are lenient if the esrog maintained its freshnes in refrigeration (Shevet Halevi 1: 176, Beer Moshe 7: 52, Oz Nidberu 13: 38).
Horav Shlomo Miller’s Shlit’a opinion is that hadassim are similar and could be used next year if well preserved

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a