> Maybe a facility is missing?
> To read in data to blend with my existing work, I must Open (which creates
> a new window), then select the addition, Cut, go back to my work data, and
> Paste the addition onto either the end of the work or a second track.

Or (assuming this is not an Audacity project), File > Import Audio...
which imports the audio into a new track in the same project.

Perfect!  Problem solved (aka RTFM).  Thanks.

Sometime I hope that we can implement an "Import Append" feature to
import multiple files end-to-end in the same track.

Yes, would be very nice.  Tony