I converted a board but i also made it shorter by cutting off the tail, which of course lost some volume. Depends how wide and long the board is, how much you need to change the shape. I could also already wing and wind foil and was just doing a project. Can be done, but not the best way to start for a beginner, you might be best to look for a cheaper foil board.

Done it to a 120ltr sailboard, worked great. It's now the local borrow board, many have learned on it for no cost before purchasing a smaller board.

90lts is very small for a beginner, you would want to be only 50-60kgs max.

Definitely, go for it, Only that the rocker of a normal windsurfing board would be slightly in the wrong place. Your feet would be placed pretty close to the place where you would stand on the windsurf board before getting going, maybe even a little bit further forward, as there is no rig weighting the front of the board down. Put long tracks somewhere where I have marked the location of the back foot, and I guess the flatter the bottom is and further forward the flat section of the bottom extends the better it would get going. I don't see why it should not work.

Yes, you can but you must refer to it as 'Frankenboard' after the transformation. We all learned on a Frankenboard with great success!

DC

Yeah but how good are the tracks in these cheap well used boards? A lot of early wing boards were crap
At least a proper conversion might last