If you want to own the copyright to the script you will need the creator of the script to agree to sign over the rights to the script, which in all likelihood will require you paying them an additional fee. The reality is that your best hope is that the creator is willing to sign over non-exclusive rights for you to make derivative works of the script.
Most designers/programmers, my self included, will not turn over exclusive rights to any code because we like to reuse code segments in other works. If we signed over exclusive rights every time we created something for a client, we would have to start from scratch every single time we started a new project, which just isn`t efficient.
I think this pretty much sums it up as I understand the concept. Pulled from here.