Hello Ravi,
In the Data Source dimension please maintain a property IS_CONVERTED and maintain that property as N for whose account members you don't want do the currency conversion...
or maintain rate type as AS_IS for those account members you don't do the currency conversion for the account members which are already converted...
And then maintain the AS_IS under formula column in the currency conversion table...
Regards,
Saida Reddy Gogireddy