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I see that Supro came out with a 20db boost that can be controlled with an external expression pedal. Any thoughts on how to convert the FET boost with a jack to achieve this option?

Thanks!

A late reply I know, but no-one else has tackled it so here goes -

The simplest thing to do is just run the boost output into an off-the-shelf volume pedal. The only thing to watch out for is that (I assume you mean the BYOC MOSFET boost?) you don't want to load the output of the boost, so you need a relatively high impedance volume pedal. Ernie Ball do a 250K I believe. The output of the MOSFET boost is from the 5.6K drain resistor, which drives a 100K level pot with the wiper as the output. So the output impedance of the pedal will vary from a few K up to a maximum of 25K as you turn the output level.
All Supro have done is insert a tip ring sleeve jack into the output of their circuit - with an expression pedal plugged in the output signal is switched, using the switch contacts on the jack socket, to cascade through the expression pedal as well as the output level on the Supro pedal. The Supro pedal output level sets the maximum output and Supro have added an internal trimmer from the ground end of the expression pedal (some expression pedals already have a minimum offset control) to the Supro pedal ground, so you can set your minimum level for the expression pedal. On other Supro pedals they have connected the expression jack to cascade other controls - tone on their Fuzz pedal for example.