How can I convert features of a layer from POLYGON to MULTIPOLYGON? I know how to do that using the postgis function ST_Multi but how could I do to same thing for a layer in QGIS?
If you want to do it based on a field, you can do this in QGIS from the menu: Vector-->Geometry Tools-->Singleparts to Multipart (requires at least two poylgons to share an attribute that you specify).
There is a more direct equivalent to ST_Multi in OGR. I didn't find a way to access this specific OGR functionality through QGIS, but that could be done using GDAL/OGR like this
ogr2ogr -nlt MULTIPOLYGON multipolygon_output.shp polygon_input.shp More details on -nlt switch are available on the ogr2ogr page.