Comma-separated values (CSV) is stores tabular data in plain-text format. To give Excel an instruction how to format a particular column you would need to user Excel format. In order to achieve it, you may use a Java library to export data in Excel format. One example of such a library is Apache POI - the Java API for Microsoft Documents (http://poi.apache.org/).
In addition, to work better with CSV files in Excel use import from text feature. This is a wizard you can specify the import settings, including column formats, width of the fields etc.
I hope it helps.