We've customized the list of activity types for Alfresco Share with our owns.
This was done by defining a module customization following this blog post :
Hence we added our custom types in the activity-list.get_en.properties file in a sourcePackageRoot, and everything output correctly in the Activity Dashlet.
But the email summary "Recent activities" doesn't display the custom activities correctly, it seems that it's based directly on activity-list.get.properties like describe here:
This email feed should take the customization/i18n files into account, and we should not have to override the file itself.