How can I prepend and append to each member of an array?

For the record, with zsh, there's the ${^array} operator that turns on brace-like expansion on the elements of the array. So:

$ a=(one two three)
$ b=('foo '${^a}' bar')
$ printf '<%s>\n' $b
<foo one bar>
<foo two bar>
<foo three bar>

Search and replace also works in zsh.

$ printf '<%s>\n' ${a//(#m)*/foo $MATCH bar}
<foo one bar>
<foo two bar>
<foo three bar>

As well as printf -v on an array:

$ b=(); printf -v b 'foo %s bar' "$a[@]"
$ printf '<%s>\n' $b
<foo one bar>
<foo two bar>
<foo three bar>

Your echo ${CATEGORIES[@]/(.*)/foo \1 bar} would work in ksh93 if written as:

$ printf '<%s>\n' "${CATEGORIES[@]/@(.*)/foo \1 bar}"
<foo one bar>
<foo two bar>
<foo three bar>

Depending on what your ultimate aim is, you could use printf:

$ a=(1 2 3)
$ printf "foo %s bar\n" "${a[@]}"
foo 1 bar
foo 2 bar
foo 3 bar

printf re-uses the format string until all the arguments are used up, so it provides an easy way to apply some formatting to a set of strings.

p='* "foo  '
s='  bar $USER' 
CATEGORIES=(one two three four)
CATEGORIES=("${CATEGORIES[@]/#/$p}")
CATEGORIES=("${CATEGORIES[@]/%/$s}")

paste <(printf '[%s]\n' "${!CATEGORIES[@]}") \
      <(printf '%s\n'    "${CATEGORIES[@]}")

Output:

[0] * "foo  one  bar $USER
[1] * "foo  two  bar $USER
[2] * "foo  three  bar $USER
[3] * "foo  four  bar $USER