You can't.

Use another column in the database to contain the product of the number1 and
number2.

Why use Word as a calculator when you have a spreadsheet?

In addition to Doug's response which describes the simplest solution, all
other things being equal,
a. if you are using Word 2002 or later you can use Word's Mailmerge events
to fire a VBA routine immediately before each record in the data source is
merged. You can use the event to insert the value you want in a number of
ways, e.g. by setting the value of a Document Variable and using a {
DOCVARIABLE } field in the document.
b. you may be able to "add columns" to your data source using an SQL SELECT
statement in an OpenDataSource routine (if you are getting your data from
Excel via ODBC or OLEDB)
c. you can use a DATABASE field in your main document to return the results
of Jet SQL functions, but you typically have to have an additional file to
do it.