sed '/Order allow,deny/ aAllow all' < yourFile
This will output the modified file to stdout. If you want to modify it in place then (sed(1)):
-i[SUFFIX], --in-place[=SUFFIX]
edit files in place (makes backup if SUFFIX supplied)
Explanation:
For each line that matches `/Order allow,deny/`:
Execute command 'a' (append) with 'Allow all' as parameter
http://grymoire.com/Unix/sed.html is an excellent resource to learn more about sed.