As you say, a Salesperson is a res.users record. It doesn't have to be an active one, though.

For example, if you have a Sales Order created manually in Odoo, assign a Salesperson other then yourself and the archive that res.users record; the Sales Order will remain linked to that Salesperson.

That being said, you could sync your ZOHO users to Odoo as inactive res.users records and assign them upon creating the sale.order record. This works because the active_test is not applicable in this scenario and what ever value is set on user_id will be be displayed.

Since these users are not secretly using Odoo by other means but you just seem to have a dedicated back office team handling incoming Orders (at least that's how I understood your case), I don't see why this should not be a valid approach within the scope of your Enterprise license. You may want to check back on this with your contact person at Odoo, however (https://odo.com/help).

Use a connector (like Webkul) for out-of-the-box syncing with salesperson control.

Or build a custom webhook + Odoo controller, mapping Zoho owner to Odoo user.

Alternatively, internal logic using automations or server actions can assign salespersons if the user is already tracked in Odoo.