You can turn any existing notes into tuplets;
In Write mode, select the notes on a single staff that you want to turn into tuplets.
But this is not possible for a the same selection on multiple staffs? Is there a workaround to do this?
I just tried different ways, but I guess, you are right: this seems not to be possible. But maybe some forum guru knows a way?
No, at the present time this works only one a single instrument at a time.