mailmerge

You have to apply the formatting in Word.

Right-click a merge field in your merge document.

Select Toggle Field Codes from the context menu.

You will see something like

{ MERGEFIELD FieldName \* MERGEFORMAT }

For a number, change it to look like for example

{ MERGEFIELD FieldName \# "$#,##0.00" }

For a date, change it to for example

{ MERGEFIELD FieldName \@ "MMMM d, yyyy" }

Press F9 to hide the field code and to update its result.