You could make an if statement activated by a certain variable. You could i.e. make the form submit to 'index.php?maillist=1', and then make a:
if ($maillist == 1) {
insert code to put the mail into a database here
}
and the main site will not be tuched by anything, except if it's called by this variable.
You could also insert a hidden field, makeing you able to hide the '?maillist=1' part, and submitting the variable directly