I don't believe you can, by design. There's even a clause in the T&C of most/all PC's that reads "cannot be combined with any other certificate, voucher, offer or promotion."

Perhaps you had been reading about various combinations of CDPs with PCs, which can stack up for a nice overall rate.

The issue there is that some PCs only work with a specific CDP. For example a AAA PC that only works with a AAA CDP, which depending on your other CDPs may not be a good deal.

Then we get excited when a AAA PC is isssued that does not have that restriction: http://www.flyertalk.com/forum/18926259-post294.html

And as you can see on the Hertz site there are also occasionally (very occasionally, and not common at all lately) Rate Codes which can be stacked in theory, but in practice I've always found to override either the CDP or PC, making it effectively not stackable.

(As for Convention Number and Voucher Number, I've never seen them discussed here.)