Word is not Excel and doesn't have the formatting functionality that Excel has.

In the number entry cells you will need to do the currency formatting yourself unless you are going to use a macro to modify the text after you enter it. Are you open to using macros in this document?

For the field you can add formatting switches to make the result formatted. The field code might look like this
{ =SUM(ABOVE) \# "$#,##0.00;($#,##0.00)" }

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia