The additional banks are:
Allied Irish Bank
Bank of Ireland
Clydesdale and Yorkshire banks
Co-Operative Bank
Northern Bank
Santander UK
These banks make up 10 per cent of the total interest rate hedging products sold to small and medium-sized businesses in the UK.
The FSA said it had not identified any mis-selling among the banks but by agreeing to join the scheme, they were ensuring customers would be treated consistently regardless of who they banked with.

The banks join Barclays, HSBC, Lloyds and RBS-Natwest who agreed to take part in the scheme on 29 June.
Clive Adamson, director of supervision in the Conduct Business Unit, said: "This is a major exercise but one we hope will ensure even more businesses benefit from having their individual situation reviewed.
"I am pleased that Allied Irish Bank (UK), Bank of Ireland, Clydesdale and Yorkshire banks, Co-operative Bank, Northern Bank and Santander UK have agreed to join the larger banks in reviewing their past sales.
"Although the number of their sales was smaller and while there is no presumption that mis-selling has occurred, it shows their willingness to do the right thing and ensure their customers who bought these products can be confident that they will be treated on an equal basis."