Audio Compositor was a shareware application which supported converting SF2 <-> DLS level 1 and 2, but its author's web site disappeared from the Internet years ago. The last version was 4.4, available as an ac44.exe installer. There is also a keygen floating around for version 4.2, which still works with 4.4.
Open ModPlug Tracker also contains code to convert the internal structures of SF2 to DLS, since its own MIDI to module converter uses DLS internally. Although I would be more interested in the reverse of this operation, or possibly even convincing Ian Luck to add DLS support to BASSMIDI.