I want to use activity watch, but I want all my stuff in one place and in the same format, so until there’s a way to import from rescuetime (even if it’s indirect), it seems like more effort than it’s worth 
I’d be interested in giving implementing this a try - however I don’t have RescueTine Premium, which you need to export your data.
Is there any chance you can provide a sanitized/anonymized export?