The problem with proprietary formats is that either the company who makes them has to release a binary module of some sort, or source code, or someone (or group) very committed has to sit down and work out the file format by hand.
So first of all what kind of format are you talking about?
You could try asking the company for a binary (doesn\'t often work, athough if they already do a Linux version it may be easy enough for them to do it), or for source (never going to happen ;-)), or just sit down with a hex editor and the decrypted text.
Si