Several of us have fabricated suspension frames from "hard tail" frames. It's a labor of love, because no one is going to spend the money it actually takes to do the work in general. Not when you can buy the latest crap from China for a fraction of the cost. Also, the best donor frames for swing arm additions are those with the reverse curve at the back of the frames. Like Bonanza and Trail Horse- both of which also made swing arm models.
Here's a link to a fabrication job I did a few years ago. I'd do it again, and it was a nice learning experience.