Using parameter expansion to generate arguments list for `mkdir -p`

setopt histsubstpattern extendedglob
mkdir -p /tmp/foo/*(#q/:s_%_/bar_)

This is extended globbing that has a quiet glob flag that uses a glob qualifier to match only directories and a modifier to perform a substitution (using the % pattern character that is only available in history substitution pattern mode) that appends a string to each word.

man zshexpn

Sure - use a loop

for n in /tmp/foo/*; do mkdir "$n/bar";done

globs are used to expand lists of existing items, not things which have not yet been created.

If the directories are in an array, you can use the ${^...} form of expansion.

mkdir -p ${^a}bar