Of course, GlobalMapper will make OSM raster-based maps with just a couple mouseclicks by connecting to their WMS servers.
If you want to use a vector based (.img file) map along with one of Garmin's .kmz "custom maps", it shouldn't be necessary to use a transparent background. The default is for vector polylines and points to be drawn on top of the .kmz maps. So roads, elevation contours, streams and POI would show. But polygons are hidden, so the map background, lakes and parks would not show.
Awhile ago Garmin added a draw order control for custom maps that I believe will now provide more control over this, but I haven't made any .kmz maps for a long time so I haven't tried it.