Obviously this is easy to do with a custom XUL add-on and some basic Thunderbird chrome scripting.
This can also be done with the Custom Buttons Add-on for Thunderbird. Here is the code for the button, you will need to replace with your own folder URI's. The URI's for the folders can be accessed from the right-click content menu Properties .
/* Code */
// 1. mark the message as junk
goDoCommand("cmd_markAsJunk");
// 2. copy the message to special folder common to all email accounts
var copyUri = "imap://{user}@{host}/{path}/{folder}";
var copyFolder = GetMsgFolderFromUri(copyUri);
MsgCopyMessage(copyFolder);
// 3. move the message to per account junk folder
var moveUri = "mailbox://{user}@Local%20Folders/{spam folder}";
var moveFolder = GetMsgFolderFromUri(moveUri);
MsgMoveMessage(moveFolder);
The button script definitions are stored in the file:
{thunderbird profile}/custombuttons/buttonsoverlay.xul
You may be able to update this file automatically to ease deployment.
WARNING: Thunderbird 31 requires different code.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1043819