I don't really understand what you are asking.
You have a mortgage in your sole name on a property in your sole name.
Your wife has a mortgage in her sole name on a property in her sole name.
You want to switch your mortgage from Bank of Ireland to KBC Bank?
I presume as it's the family home, you might need her agreement. But she does not need to be on the mortgage.
"I then took a loan (home loan) to build an extension. This is at 10%"
I assume that this is not part of your original mortgage, if by 10% you mean a 10% interest rate. So you want to extend your existing mortgage to pay off the CU loan for the extension?
Brendan