Adding co-owner before flat registration and after loan

Hi,

My wife and i had bought a Flat in bangalore before marriage and the flat is in my wife's name.The loan(HDFC) and every thing is in her name .The property is in construction phase so registration is pending for it.

Now i and my wife want me to be added as co-owner in the property.The builder says the initial agreement has t change but Bank say they cannot give back the original agreement for modification.

Could any one suggest what will be exact process ?

Thanks in Advance !!