It is not possible possible to add Document Action button as a list view button, since Document Action must have some Base Id to start generation from – you would receive an error message “invalid ID.”  Each document will be generated per each Task record, so as the workaround you can create a list button that will edit all records in the list and this edit will invoke RS Document generation script. If you would like assistance to develop this feature, our consulting team at Ramsey Solutions could assist, submit a case to Support to get started on your quote.

You can however still use Task as your base object API though you need to add the actual custom button on the Task page layout, meaning the button needs to be assigned to a specific record type.