It seems that there were some problems with older hints to import and export signatures to and from Mail.app. In response to this Macosxhints forum question, I wrote some scripts to help with the process.
Since Mail.app uses .webarchive files (stored in your user's Library » Mail » Signatures folder), the scripts use the command line tool textutil to do the conversion from RTF to webarchive. textutil can convert to and from other formats you may want to use as well: txt, html, rtf, rtfd, doc, docx, wordml, odt, or webarchive.
Import_Mail_Signatures Script -- View the script's source
This script will allow you to choose a folder of RTF files, convert them into the new webarchive files, and place them into
the Mail.app signature folder as described above. It will also write the details of the new files to the SignaturesByAccount.plist
(also in the Signatures folder). I would suggest you quit Mail.app first before running the script. But if you don't, you can quit Mail.app after you run the script, then reopen Mail, and Mail.app will read the modified plist file.
I have only set the script to put the new Signatures into the All Signatures preference (in Mail.app's Signature preferences).
All you have to then do is drag any of the signatures onto an account name as normal within the Signature preferences. You can remove them if you wish, using the options to delete in the same Signature preferences panel.
Export_Mail_Signatures Script -- View the script's source
This script will match the Signatures found in the AllSignaturesKey array of the SignaturesByAccount.plist file with the webarchive files found in the Mail/Signatures folder. Any Signatures found in the AllSignaturesKey array will only be processed if a webarchive file is found for it.
This script will then convert those matching signatures into new RTF files and place them into a folder of your choice, naming them as the signatures are named in Mail.app.
Extra Script
This script is just to show you how you can use textutil with Applescript to convert to and from any of the supported formats of your choice.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080419202652327