Is it ok to fasten a shower surround to Schluter Kerdi curb?
The kerdi curb won't hold your screws.  You can try putting a screw on a piece of scrap and you should be able to pull the screw out by hand.
It is designed to be used at the bottom of the shower door just to hold the tiles and good enough for a person to step on it.  The side hinge of  the shower door must be mounted to a stud.
Kerdi curb and boards are not substitute for the framing.  They are like drywall+kerdi membrane all in one to reduce installation time.
To seal the screw holes, drive the screw in half way, then squeeze some kerdi fix in there, and drive it all the way.