SSIS imports NULLs instead of numeric values from an Excel file

Let's imagine that you have been assigned a task to import some kind of parts catalogue from an Excel file to SQL Server table. You decided to use Integration Services. One of the columns in the Excel file contains catalogue numbers. Most of them look like this XYZ123456 but some of them are plain numeric values.

This is what can happen:

Excel          SQL Server
CAT_NO         CAT_NO
---------      ---------
XYZ121156      XYZ121156
XYZ654321      XYZ654321
ABC120456  =>  ABC120456
CBA183456      CBA183456
ZYX123416      ZYX123416
123            NULL       (!)
ZXV654521      ZXV654521

This article from SQL Server 2005 Books Online explains this weird behaviour:

Missing values
The Excel driver reads a certain number of rows (by default, 8 rows) in the specified source to guess at the data type of each column. When a column appears to contain mixed data types, especially numeric data mixed with text data, the driver decides in favor of the majority data type, and returns null values for cells that contain data of the other type. (In a tie, the numeric type wins.) Most cell formatting options in the Excel worksheet do not seem to affect this data type determination. You can modify this behavior of the Excel driver by specifying Import Mode. To specify Import Mode, add IMEX=1 to the value of Extended Properties in the connection string of the Excel connection manager in the Properties window.