Yes it is. I mean, you should be able to do it later, but if you have to ask how it might be better not to risk it since that would involve resizing your root partition. If however you have a secondary disk you want to use for it it’s just a matter of adding a new entry to /etc/fstab (which the UI installer in Mint and others allows you to do with a nice UI instead of having to write the file manually). Although bear in mind that mounting a disk on top of an existing folder will mask the contents of the folder, so you won’t be able to access the files that were there before. Long story short you can move the files over to the new disk first, them add the fstab entry and it should work, for future installations you can set this during the installer, same way you create a different partition for /boot, in Linux partitions are mounted to folders and they work as if they were in that location for all intents and purposes.