Yes. You need a program to calculate the surface, for example VOIDOO (which will directly create an EZD or MASK file). Mappage the file or convert to ODL instructions. If you have mappaged your surface, you can contour it just like you would do with electron density.
On the SGI version, you may in addition use the semi-transparent surface rendering capability of O. In order to use this, do a map_par and select line type 5 (note that only a few colours are supported in this mode, such red, yellow, green and blue).