Yes, you can paint over K-Rend with Sandtex and any other brand of regular masonry paint.
And, no, it's never a good idea to add PVA since the paint will have sufficient adhesive qualities already. PVA can also create a film which is detrimental to the breathability of the paint finish.
If this is a recent application you should contact K-Rend directly for advice in case this invalidates any current warranties.