Dear colleagues, is there a way to add a custom field to multiple existing projects at once?
No, unfortunately, you can only add custom fields to one project at a time.
A very narrow use case that might help: If you have Asana Business plan, then in Workload you can Edit Effort and add a Numeric type Custom Field to all Projects in the Portfolio.
Another related suggestion: If you add the field to a Template it will be easier to create new Projects possessing the Custom Field.
Maybe one of those helps in the absence of anything else,