If you are using the standalone mail client, you can assign a keyboard shortcut as Pesala said (if the forum hadn't run all his code together).
Read mail, 1
Go to Preferences > Advanced > Shortcuts, click on Edit, select a context that you want the shortcut to work in (usually [Application]), click on New, enter a key to use for the shortcut (maybe U?), press Tab and in the right column type the command above, then click the Okay buttons in both dialogs.
If you want a button, you will have to edit a file.
Edit "C:\Users\USERNAME\AppData\Local\Opera Mail\Opera Mail\accounts.ini" in a text editor while Opera Mail is closed. emc's 11.50 suggestions should still work. The lines (if they're not there, add them) are under each account's section. See http://www.opera.com/help/tutorials/mail/advanced/#custom-reply for what you can do.