isn't this against all experience?
just count how much characters you can enter in a company code field in SAP, it is 4 and not 10. So why did you add 10?
The message that the source field is longer than the target field is very helpful, as it tells you about a possibility of truncation.
The source field should reflect the maximum numbers of characters used in any of the cells for company code in your source file.
If that is really 10, then you will miss the last 6 characters when the source file content is moved to the target field.