PgAdmin 1.9+ has a simple Plug-In architecture which makes it relatively simple to introduce new plugins. It is pretty much all controlled by the file plugins.ini. In that file you can register any executable you want in there. If you want the executable to get passed database configuration settings, there is an option for that and you just have to build your executable to accept commandline switches.
You can download the windows version from http://postgis.net/windows_downloads. If you just want it without the PostGIS 1.5 binaries -- just download the one labeled PostGIS ESRI Shapefile GUI.