The Iso8583 is a complicated standard for financial transaction card originated messages, mainly used in EFTPOS or ATM networks.
Although this is a standard there is no real standard implementation, as each network customises the messages for its own purposes. There are 3 major versions Iso8583-1:1987, Iso8583-2:1993, Iso8583-1:2003. It uses bit mapped fields (1 or 2 64bit words) to define what subsequent fields are included within the message,
To further complicate matters you can use ASCII or binary (BCD) encoding.
To use this within Biztalk you should probably write your own custom disassembler/assembler that would sit within the pipeline and convert incoming Iso8583 messages into Xml messages and vice versa.
I have written a C# library for serializing and deserializing Iso8583-1:1987 binary messages, but this was not used within Biztalk but used in a WCF service. The service accepted Xml messages, serialized into Iso8593 and passed to a payment processor. The response was deserialized into Xml and returned to the web service client.
Are you able to give any more details on the version and encoding for your Iso8583 messages?