SecuMail is associated with the ASC extension, but we have no control over the Mail app, so we cannot do anything "automatically". We *wish* would could write a "plug-in" for doing stuff directly in Mail, but doubt that they will ever allow that.
That being said, you can easily open the ASC files (no copy and pasted necessary). Just tap-and-hold on the file attachment icon. This should bring up a box that has an option to Open in "SecuMail". Tap on that and it will launch SecuMail and prompt you for your password.
Hope that helps?