Instruments in Score pane

Not at all.
You can add instruments only in the main score.

Adding existing instruments to a Part doesn't work either, you'd have to add a new part containing the instruments you want

In the Parts dialog box use the middle button

I think it's possible to add a new instrument in a created part, by saving this part, alone, in a new file. After you load iti alone , and you can add, but it's not linked with the score