Customer as Supplier

 hi experts

I have a small doubt. Would like to have your advise and guidance on that.

Can we create a Customer already existing in AR as a Supplier in AP.

I am making a refund to my customer which creates an invoice in AP. But the supplier number and supplier site fields are not populated in the invoice. These two fields are mandatory fields for one of my customizing requirements.

please offer your opinions on this.

regards

gm