What kind of condition. I remember being able to use CASE statements in control files. However, that was over 3 years ago, so the exact syntax I don't remember. There is always the ability to load the data raw into a holding table and do a query on that table and also use conditionals. But you should be able to do that in the control file.
I know there is a SQLLDR guide online from Oracle on control files and it has everything you can put in them.