I'm not sure that you really want to use for this is a RIN number. Very often the RIN is used internally within the software as a database key for record relationships so the RIN is usually controlled by the system and is unique only to the system/database. The RIN can sometimes become the XREF ID in the GEDCOM. It would not be a good idea to mess with this.
Where are you doing the sorting? 1) Within your genealogy software. 2) in an extracted spreadsheet. 3) other program.
I assume you want to enter the value yourself when you add the new individual.
The GEDCOM definition provides for an "REFN" tag know as a USER_REFERENCE_NUMBER and defined as: "A user-defined number or text that the submitter uses to identify this record. For instance, it may be a record number within the submitter's automated or manual system, or it may be a page and position 64 number on a pedigree chart."
Setting up software to allow for the entry of a tag that is part of the GEDCOM standard should not be a problem for most "good" programs. I can do it with mine.