I assume you are using the database import feature?
The import will attempt to guess the appropriate format for the column - in this case numeric. If you run the import and then change the field definition to Character - which a customer number really should be anyway for practical purposes if yousee what I mean - the null field should be correctly populated.
Save the model with the new field properties and then apply that next time you import from the same spreadsheet and all shoul be well.
If the Excel file name is always the same you could save the whole thing as a Project so that it all loads up from one place next time.
I hope this helps.