I plan to build my own touring SUP by “enhancing” a wind surfer board. The board is made of glass, 12’ by 30 “. I plan to add 4” to the width, 3’ to the length, 2" to thickness. I will attach a wood frame to the wind surfer board and marine plywood and glass it – and i will try to follow the same lines as the wind surfer board.
Will this work? Or not. And why not?
weight
That sounds like it will be extraordinarily heavy.
Yeah
yeah, i was thinking the same thing …
How about this?
The other thread here about using a canoe as a SUP, your thread for using a surf board to DIY a SUP, and the fact that I have a stripped fiberglass shell of a canoe waiting for gunwales and some fiberglassing taking real estate in my backyard for 2 years now, just gave me an idea: remove 10" or so of the canoe sidewalls down from the gunwales, add some stringers along the length of the hull, get some pink construction foam to shape a deck and glass over it.
Effectively, it will convert a useless canoe hull into a potentially useful SUP…
I do wonder, however, why no one shapes SUPs like flattened canoes - probably because they won’t paddle nearly as well as a purpose-built SUP hull. Or maybe SUP builders have not yet discovered how “optimized” canoes are 
Sailboard
Just throw it in the water and go. I finally got around to trying SUP on my old Fanatic Fox the other day, used one of the oars (alumionum/plastic from West Marine) off the sailboat for a paddle. For you windsurfers who are not old enough to remember it, the FOX is a 12 ft long board around 200 liters, plenty floaty, and 26" wide, which seemed like a lot at the time, but much narrower than today’s boards. Without the sail to steady it, I felt really unstable, and went in the water once. Paddled really easy though. If your board is 30" wide it should be good to go as is, lots of SUP boards are around that width. Bic makes a board for SUP and sailing, which I would consider except it doesn’t have footstraps.
Try it and let us know how it goes.