In FileMaker Context, "Importing" implies that you are using Import Records to import multiple rows of data into a table in your file. Is this what you are doing?
If so, you can define the following auto-enter calculation on the field:
If ( GetAsText ( Self ) = GetAsNumber ( Self ) ; self ; 0 )
Be sure to clear the "do not replace existing values..." check box.
You can then enable auto-enter options during your import and those rows of data where this field receives non-numeric data will be set to zero.
If you don't want to or can't enable auto-enter options, you can use a very similar calculation with Replace Field Contents to update the values in this field immediately after importing the data. To do this with Replace Field Contents, you'd replace 'self' with the name of the field.
Thanks very much. That is exactly what I needed.