使用SSIS从SQL Server 2005中的平面文件导入时如何保留NULL值

I've exported records to a flat file delimited by "|" and it seems that when I import those records into a new database , SQL Server treats the NULL values as empty fields. IMy queries worked properly when the records/fields were NULL and so I want to either find a way to retain the NULL values in the data or convert the blank fields to NULL values. I'm assuming the former would be easier, but I don't know how to do that. Any help would be appreciated.I've exported records to a flat file delimited