Making a mainstack subordinate to another stack

[Mikey]

I have a mainstack (with substacks) that I would like to make subordinate to
another stack.
The correct procedure would be....?
set the mainstack of blah to goo doesn't do it since blah has substacks.

-- 
On the first day, God created the heavens and the Earth
On the second day, God created the oceans.
On the third day, God put the animals on hold for a few hours,
and did a little diving.
And God said, "This is good."