What is the exact procedure for creating more Ripple currency units (XRP) in excess of the default 100 billion that already exist?
There is no such procedure. While OpenCoin could accomplish it today by changing the code to create such a thing, in the future, the procedure would be as follows:
Propose some kind of change that would create more XRP.
Assign it a 256-bit ID. (It can be arbitrary.)
Release patches that support this change, and announce their support for this ID.
Get 80% of validators to run your patches.
Wait out the two week waiting period for new feature enablement (to give a chance for review and vetoing).
If 80% of validators (weighted by trust) still run your patch, the feature will enable itself and then you can create more XRP. Alternatively, you could try to convince about 50% of validators (weighted by trust) that your patch is so urgently needed that the normal supermajority delay process can be bypassed.