Command to install spf record on single cPanel account.
# /usr/local/cpanel/bin/spf_installer cPanel_username
Put actual username in place of cPanel_username
Instead of installing spf records on cPanel accounts one by one use the following script to install it in one step:
# cd /var/cpanel/users
# for i in `ls /var/cpanel/users` ;do /usr/local/cpanel/bin/spf_installer $i ;done