Assuming that you use Windows XP, Vista, 7 or 8/8.1, all ZIP files can be accessed as if they where simple folders directly from the explorer.
On a Mac, you probably have a similar functionality. The same apply to Linux.
In Thunderbird, you have a "Sent" folder where a local copy of e-mails that you sent are stored. Find the message that you want and drag it to the "Draft" folder.
From there, right-click the message and sellect "Edit as new".
You can edit the message or change the recepients any way you want.
Also, Thunderbird automaticaly save the e-mail addresses of messages that you sent to the contacts list. Hit the F9 key when composing a message to open the addresses sidebar from where you can add an address by doubleclicking on it.