No, it's not possible. However, if you have access to the server with the partial KMS 2012 R2 key, what would be useful is if you could make a copy of the C:\Windows\System32\spp\store\2.0 folder. (It should contain data.dat and tokens.dat; data.dat is a hidden file.) These files, together with the partial key, are useful.
Really? Just really? Wow... I wouldn't say "not possible" if it wasn't completely, utterly impossible. Like, "walk on the surface of the Sun" impossible, as opposed to "maybe one day someone will figure it out" impossible.
This reminds me of the "good old Atari" days. Back in the 80s, there were a lot of magazines that had type-in Basic listings. After a while, checksum programs were used to check every basic line and produce a 3-letter checksum. Of course, some were wondering if you couldn't just type in a 3-letter checksum, and have "some smart program" turn this "back" into the full line. Seems logical, no? I believe they even printed a listing of a program that was supposed to do just that, but instead, it printed something like "Happy april fool's"... (I even looked through my old Compute!, Antic and Analog PDF's, but can't find it...)
Well I have to admit that the "decode" part of my question it's really imposible like Atari already said. But when we are talking about generate, it's just maths and time, so at the end possible. If some1 manage to create a program to generate random letters/numbers in the 20 first position of the key and check the 25 generated key against slmgr it's possible, but probably it will take a lot of time.