Will it be possible for PC users too transfer the Destiny 2 folder to the Steam common folder where games are installed, so the only thing we have to download is shadowkeep or will I have to re-download the entirety of D2
Officially, that won't be supported. All the binaries are going to be different as they need to switch from Battle.net frameworks to Steam frameworks. Also, the Data that you will be downloading from Steam will be significantly different than what we have on Battle.net, as it will contain all the changes to New Light and the new Shadowkeep content. I imaging some things might be the same, but you are always going to be looking at a large discrepancy between the two installations.