Depth of field tables for the Dimage 7 series can be found here:
http://www.minolta.cz/uploaded/download/d-7_depth.doc
One of the characteristics of digital cameras -- with CCD's smaller than 35mm film -- is a large depth of field. Some praise it and some curse it, but it's just a fact of life.
For smallest depth of field use maximum zoom and widest aperture. A teleconverter will also reduce depth of field, but there aren't any that don't vignette at less than maximum zoom.
Close-up lens won't help unless your subject is... CLOSE UP. I just tested one to make sure. The camera can't focus on distant objects with a close-up lens in place.