Adding custom action

I am using iThemes and I would like to know if it is possible to add a custom action (in the functions.php file within the theme folder) to add particular behaviour.

For example, the plugin allows to authomatically ban IPs when someone attempts to login as admin.
Looking at the plugin log I’ve noticed that someone is trying to login using some odd usernames, always the same. One is not a registered username onto my site, another yes. So I would like to authomatically ban IPs using these usernames in the following cases: always if the user is ont registered, everytime the login fails if the user is registered.

I know that it is not a common feature and even not advisable, but my website has a few members, and they are not going to change in the future, registration is not allowed. I am even ready to pay the consequences of some extra work.

Thanks