Filename read during HL7 message processing

I need to read first six characters from the filename we process in healthconnect, then need to insert these six characters in MSH-6 in the same HL7 message. (please note each file can have multiple HL7 messages with multiple MSH segments)

Is something can be done when first process the file, then insert first characters MSH-6?

Please advice if this is an easy fix.