You will need to create a custom module with hook_form_alter() implementation.
function [YOUR_MODULE]_form_alter(&$form, &$form_state, $form_id)
{
if($form_id == "YOUR_FORM_ID")
{
// target the submit button and add a new submission callback routine to the form
$form['#submit'][] = 'YOUR_SUBMISSION_CALLBACK_FUNCTION';
}
}
The code above will execute your new callback function YOUR_SUBMISSION_CALLBACK_FUNCTION
AFTER the form's default callback function.
To make your new callback function called BEFORE the form's default callback function, use the following code instead of the giving above:
array_unshift($form['#submit'], 'YOUR_SUBMISSION_CALLBACK_FUNCTION');
To cancel the form's default callback function and force it to use your function ONLY, use the code below
$form['#submit'] = array('YOUR_SUBMISSION_CALLBACK_FUNCTION');
Hope this help.