provide separate volume and pan controller for the voice of a stave

See also #58031: Dynamics/hairpin range for Voice
Or use the attached template and it's channels, which is what I use for exactly the same purpose as you intend, biggest advantage: works in 2.x already ;-)

I manually edited the mscx inside the mscz to add those channels, and yes, you could do the same changes to your existing scores.

Nope, plugings can't do that. Exctact the mscx from the mscz either by opening in MuseScore and sava as mszx or by using e.g. 7Zip. Open the mscx with some plain text editor, like vi or Notepad++
Locate the term Channel and replace

        <Channel>
          <program value="52"/>
          <synti>Fluid</synti>
          </Channel>

with

        <Channel name="Soprano">
          <program value="52"/>
          <synti>Fluid</synti>
          </Channel>
        <Channel name="Alto">
          <program value="52"/>
          <synti>Fluid</synti>
          </Channel>
 

resp.

        <Channel name="Tenor">
          <program value="52"/>
          <synti>Fluid</synti>
          </Channel>
        <Channel name="Bass">
          <program value="52"/>
          <synti>Fluid</synti>
          </Channel>

Then add staff text to either staff and via right-click/stafftext properties assign voices to these channels

Is it possible to do it using a modified instruments.xml file? If so can you explain how to make it work?

Yes it is, see above and apply this to the Men and Women 'instrument'

It indeed might. I have a PR that adds more channels to several instruments (https://github.com/musescore/MuseScore/pull/3500), this here should/could be part of it.
Problem still to solve: in Mixer show only the channels that are actually used, in order to not clutter up the mixer dialog.

At least for the 'Instruments' "Men" and "Women" and the Closed Score SATB templates this issue is solved meanwhile, by providing channels, as shown above

see also #278836: Allow for muting voices in mixer and #278881: Restore buttons for muting individual voices within a staff

? You mean Men/Women/Closed Score SATB? These certainly work for me, in Beta
And for 2.x scores that used the above 'tricks' work in 3.0 too (and ever did)

It is not by voice though, but by channel, and it is up to the user to assign voices to channels via a stafftext and the staff text properties

Where in 2.x do you see voices in the mixer?
The have been earlier 3.0 developer builds that had that, but never 2.x

Do you mean that template from furter up? Opening that with 3.0 shows 2 instruments with 2 channels each, just like it should, and just like it does in 2.x (with a different UI though

And this is what I get in 3.0:

After pressing on these arrows...

The big change in the new mixer is that channels by default are not shown, so don't add clutter

PSA: it's not only the channel switch that jojo did in his file, he also added a Staff text (CTRL+T) and changed the properties to distribute the voices to the channels.

Indeed, those staff texts are responsible for that channel switch. And they are available in the text palette (of the advanced workspace)