PM send-items on/off option in User CP

[Black Tiger]

Seems to me this must be possible by doing some template changes and make a user profile option or something.

This is what I mean.
At this moment in the usercp when you reply to a pm the following option is selected automatically:
Save a copy of this message in your Sent Items folder.

Now I've editted a template so this is automatically not-checked.
But some other users are complaining again, because they would like it checked by default. Others do want it unchecked by default.

The only way to achieve this is to make a selection box in which users can select if they want to option selected or unselected by default.

If it possible somebody makes me such hack? Since for now it is only possible to select or deselect it on a board-wide basis and not on a per user basis.