Question about Magnetoplan SP Whiteboard

Theoretically... There is no shelf for the short side. I doubt whether there is an attachment option for it at all. With a little tinkering and sawing off the longer shelf, you can certainly convert the whole thing to portrait format.

Mount in portrait format: Yes, this is possible without any problems.
A shelf is not supplied for the short edge. Simply dismantling the long edge, shortening it and reassembling it is not so easy because plastic parts and aluminium parts that are only present at the bottom have to be adapted.
However, it has a T-profile on all 4 edges towards the inside. If you are good with your hands, you can make a new one and hack it into place. However, it will not be easy and 3D printers with manual skills will help immensely.