How to make carrier available only if there is an existing order

[nikmagnus]

Hi there,

 

I'm trying to figure out how to make a special carrier available only if certain situations are true:

• if there is an existing order (paid for or not paid for)
• If the existing order status is not shipped or preparation in progress.
•  

I'm needing to set up a carrier called "add to order" for a supplementary order.

 

Any ideas on how to go about this?

Im guessing I will need to make an override and modify a controller, but I'm really  a bit lost here.

 

Cheers, Nik