I am not sure if you have got the answer you are looking for, but I have discovered this lately.
I was loading a delimited file with n columns and i had to skip columns in between, like load column 1, 3, 10 and so on... to suffice this, there is an utility in sqlldr called FILLER, you can give any dummy name and just say FILLER.
In case of a fixed file, just skip off those positions and columns in your CTL file. I hope I have answered your question.