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...