It is possible to greatly expand the chord possibilities on a DG melodeon by combing a bass note and chord from any of the available left hand buttons. This technique is known as cross-chords or cross-chording. The chart below shows the most useful and musical chord possible on a DG melodeon with the Bm variation of the standard Saltarelle layout.
All chords are designed to be played with the 3rds removed, and as such chords are referred to by their name on this layout rather than their actual harmonic sound eg. the Em chord actually sounds as a neutral E chord, neither major nor minor, but is referred to as Em regardless.
Various combinations on the chart are available on a standard 8 bass DG box such as a Hohner Pokerwork/Castagnari Tommy/Dino Bafetti Black Pearl, and as such, can be used with these sorts of instruments.
This is not an exhaustive list, but one containing chords which could be used in every day playing. Some possibilities have been omitted due to sounding discordant, eg. B bass + C chord. These could feasibly be incorporated into a tune with careful thought and planning, but generally have little use.