Is it possible to install nukes into a sub that wasn't designed to carry them? And if it is,how difficult is it?
Well if you are talking about N-tipped cruise missiles,it wouldn't be too difficult.Israel probably has such weapons on it's German built subs & most modern D/Es can launch Tomahawk like missiles from their 553 mm tubes with minimum modifications.The problem with that approach is that the cruise missiles will take up valuable (& already scarce) space for torpedoes & anti-ship missiles.The 'CKB-Rubin' bureau in Russia seems to be offering a pretty novel solution to this problem.They have offered a 8 cell VLS for supersonic cruise missiles on their modified 'Amur/Lada' class D/E subs proposed for the Indian navy.