Imploding to Grand Staff

No, there is no command for that and you cannot specify a threshold for 'Implode' (as far as I know). And I don't know if s.o. wrote a plugin.

I would suggest this approach (not tried it):

• Add a piano
• Switch to the concert pitch view to make it easier to see which wind instruments fit best in bass clef and which in treble clef.
• Sort the order of the instruments so that the instruments that will later go into soprano clef are directly below each other, and those for bass clef as well.
• Select all staves that should go into the soprano clef and implode them. They will be grouped together on the first staff line.
• Select all staves for bass clef and implode them.
• Now copy the imploded notes of the first soprano clef into the soprano clef of the piano and in the same way copy the notes of the first bass clef into the bass clef of the piano.
• Finally, you can delete all wind systems if desired.

You still can't set a threshold and may have to manually move incorrectly assigned notes to the correct staff. It is best to do this before imploding.

Addendum: You can only implode up to four staffs, because Musescore cannot handle more than four voices.
And probably you have to adjust some stem directions or exchange voices, because the default stem direction is up for voice 1 and 3 and down for voice 2 and 4.

You wrote:
It all goes into the Treble Clef. Can I automatically move the lowest notes (a threshold that I define) into the Bass Clef (via a macro or command or Plugin)...

For MuseScore 3...
Right click on an empty part of the (imploded) treble clef staff and then choose 'Split Staff'.
Middle C (C4) is set as default, but can be changed.