We have some partners that we want to be able to have access to our SFDC org so we can track things better. We talked to our account rep but he said he wasn't sure if you can add partner portal accounts to an ISV org. I'm still waiting to hear back from him but I thought I'd see if anyone here knew the answer.
We use our ISV edition as our main Salesforce org and have never had any license issues. We haven't needed to go the partner portal route, but we do have a customer portal and we just called the AE and purchased the licenses. We've also purchased a number of service cloud and content licenses in the same way.
The ISV edition is simply an enterprise edition with two free licenses (and potentially the License Manager App installed), so there wouldn't be any technical reason why this couldn't be done. Given that you will have to pay for the partner portal license I can't see why there would be any concern.