If I'm creating a new row, and I type in an Identifier #, is there a way to auto-populate an expiration date column with a date associated with that Identifier #?
Could you use a calculated column? SharePoint allows you to use Excel-like formulas conditionally populate data based on other content in your list item.
If the due date is based on a formula related to other information in your list item, or somehow embedded in the identifier #, then a calculated column should work. But, if that data lives in another list the calculated column won't be able to reference it.
If that proves to be the case and you need to do a lookup to another app, the easiest option would be to use a SharePoint workflow that automatically runs on item creation/modification and grabs the expiration date with the "update list item" function. Here's a basic overview.