Hi Klesh.
I bought KSP back in 2013, and Making History in 2018, direct from KSP. I've tried "adding" KSP to my Steam library, but Steam still refuses to sell me the Breaking Ground expansion.
So what I'm asking myself is, why should I pay again for a game I've already bought and own (if you can actually own it, but that's a debate for another discussion...)
I'll try contacting the KSP store direct and see if I get a better response than the excitable earlier poster