You need to edit the database directly.
1. In the mdl_user table find your manual user and note the number from the 'id' field.
2. In the mdl_config table find the record where name='siteadmins'
3. Change the value in that record to include the id number you noted earlier. E.g., if the id was 29 then the value will be something like 2,29 (it's a comma separated list of ids)
BTW... don't *ever* make your primary admin have external authentication - always manual.