Welcome to the Forums, Hold! Glad you're enjoying Stardew.
Good news, you won't have to do any file transfers or save rejigging, because Robin will sell Cabins even in Singleplayer.
They're really cheap, and you get the advantage of being able to place them wherever you want! Just construct the needed amount, sleep that day to save your progress, exit to the title, and open your farm via the Co-op button rather than Load.
If you have any more questions, just ask, and I hope this helps!