An endless loop triggered by a key is not possible. I think you are talking about "sostenuto" that should be made at the MIDI level: this would be a feature of your keyboard that hold on key until you trigger another one.
In a soundfont you can however loop your samples and select the maximum release of around 100 seconds at the instrument level. You will need to set an exclusive class also, so that when a key is triggered the previous one (sharing the same exclusive class) is quickly muted. www.polyphone-soundfonts.com/documentati...es/instrument-editor
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