Hi,
Mail.app is supposed be in the list, if it's not please let me know so I can find out why.
You can add applications to the workflow by adding the bundle ID to the "supportedApplications.txt" file in the workflow folder. Remember to add a comma and the filetypes the application can work with or "*" for any filetype. As an example, to add support for iSkysoft iTube Studio the line would be "com.iSkysoft.iTube Studio,*"
The workflow make use of the shell "open" command so make sure the application supports this.
I have added support for iSkysoft iTube Studio in the latest version of the workflow, you find the link in the first post in this thread.
Cheers,
Jonas