Transformation Rule to convert a positive number to its negative equivalent

I’m trying to import a bank statement file into BC and I want to convert one of the amount columns to its negative equivalent (e.g.) 569.96 to -569.96 ? In .NET I would multiply the value by -1, but how would one do this during a bank statement import. It looks to me like Transformation Rule in Field Mapping in Data Exchange Definitions might be a way. I tried to create a new Transformation Rule, but where to specify Value * -1 ?
Can this be done? If yes, how? Thanks in advance for your help.