Hey Tom,

I guess this is not the intended way, but for me it works (Odoo 13 EE):

• Create a new field for the model sale.coupon (typ: char), in this example I will call it tmpCode (x_tmpcode).

• Create your coupon program and as many Coupons as you need.

• Export your coupon program with the fields id (external id) and coupon_ids/id (external id)

• Add a new  column to the export-file, call the first row coupon_ids/x_tmpcode and add your codes you want to import, one in each line. Like this:   

• Import the file (not without testing ;) ) in coupon program, pay attention that your codes are imported to tmpCode 

• Create a new server-action for sale.coupon, Phyton Code:

for rec in records:
  rec['code'] = rec.x_tmpcode

• "Create Contextual Action"

• Go to your coupon program - click on Coupons, select all and start the created server-action

Now all the codes created by Odoo should be overwriten by the ones you imported.