You cannot do this in VCD via any API/PowerCLI/UI. Not even System Admins can set lease times longer than what the Organization settings are. I've been in touch with VCD UI developers over the past year on this and they still don't get why we need this option.
I've written a PHP application that queries the DB for vApp Templates and displays them in a table. You can then select the ones you want and set them to never expire. You could modify it so you can look at vApps as well and have some different values for changing lease times. The queries are in the PHP code. Let me know if you have any questions about it. http://veric.me/2013/05/14/set-vapp-templates-to-never-expire-within-organizations-with-vat-storage-leases/
-Eric