Most of the railing objects have a inclination angle. This adjusts the slope. Try setting up a temporary elev/section looking face-on to the railing. You can then measure the angle needed exactly. Unfortunately there does not appear to be a hotspot to adjust the slope graphicly, which would be really useful in these situations.

ArchiCAD does support sloping slabs. They are called roofs! They have identical settings to slabs but can slope, and individual edge angles can be adjusted.

The mesh-to-roof goodie may be useful to automatically convert your existing meshes. The link to the goodies is on your 'help' menu in ArchiCAD itself.

Hope that helps!