I found a workaround to do this. I had hundreds of 2d fills to convert into slabs and I didn't want to use the magic wand to achieve that.
1- Create a profile with all the 2d fills and create a column with it
2- Export it from the 3D view as a .3DS file
3- Import in Sketchup (select "Merge coplanar faces" in the import options)
4- Explode to get seperate volumes
5- Save as a Sketchup 6 .skp file
6- Import in Archicad with an old Sketchup plugin (plugin for Archicad 11 works). As a Mac user I had to use a Parallels Desktop to use de Windows version of Archicad
(newer version of the plugin create an objet which has no utility, older ones create slabs, walls ...)
7- Use manual rules to import: horizontal faces -> slabs
(you'll get twice the slabs, one for the upper side and one for the lower side - think to put a different texture for both sides of the column so that, when importing back in archicad, you'll get the duplicate with another texture. it's easier to find and select)