To enclose a string with quote pair (or its double) in a bigger string

How to enclose a string with quote pair, or its double, in a bigger string from the Bash expansion ?

Code:u=`echo fo* b* fo*b`
echo $u

$ foo bar foobar foo bar
the last is supposed to be a space contained string enclosed with quote pair, or its double, so

$ foo bar foobar 'foo bar'i.e
Quote:

u="foo bar foobar 'foo bar'"

Thanks before