Legacy Back End System

I am using the Integration Server 4.6. for EDI processing. I receive an in an EDI document and I am trying to write to my legacy back end systems which reside on a mainframe. All of my backend systems require a fixlength file, with numeric fields left justified and padded with zeros. The flat file templates does not support numerics and left justification. Does anyone else have these type of backend files? How are you performing these types of processes? Any help would be appreciated.