We have been using the vCenter client in order to create the OVFs.
This is what Studio can automate for you; there is no need to use vCenter to create OVFs. Adding on to an existing OVF is possible, but it usually ends up with something unmaintainable and not very well reproducible. Take a look at creating the entire vApp within Studio, and I think you'll have better success.
Can you guide step by step how, i can use existing OS.
Simple app like winzip to deploy.