Use Cpyf with fmtOpt *NOCHK
Assuming that the flat file is
1. In a library on your AS/400
2. Fixed length fields
1. Create and compile the DDS which represents the data as fields
if first 30 are address, next 20 are city, next 10 are zip create those fields
at this point the fields probably are all text.
2. Use the CPYF command to copy the flat file to the fielded file.
Once you have it in a fielded file you may want to use RPG or SQL to convert some of the text fields to decimal.