[otrs] supplier / partner case number

Hi, I would like to add a field where the ticket/case number of the other party (customer/partner/supplier), and it would be superb if the postmaster mail could subtract it automatically (probably requires heavy coding). The case reference field I assume could be added through SysConfig as FreeText. Been struggling with this, how to go about and add an additional field (that could be entered/modified)? Perhaps anybody is using a similar field? gr, Frans