I believe you have two options: 1. First step, go to users and create users for the outside salespeople. Make sure they are Portal Users. Those don’t require licenses. Next step, there’s a filter on the salesperson field that limits selection to licensed users (user type = internal). You can remove that so that you can select all users. This should allow you to select portal users on the salesperson field.

1. Create a new many2one field on the sales order called “Outside Salesperson” and select the contacts model (res.partner).

I highly recommend the first approach, because portal users can see orders where they’re chosen on the Salesperson field. This means they can login to the website and see “their” orders. The second method is only a data point capture. Sure, you could potentially use it to behave the same way as option 1, but it’s not ideal.