There is not a way to directly enter previous years' grades in OLLY's transcript report, however there are two things you can do:
1) Create "placeholder" lesson plans for each of your son's prior public school courses, with just one assignment in each one. First create "School Years" for each previous year you wish to include in the transcript. Create Course Names for all applicable classes your son took in those years (I would recommend including Course Descriptions because these can be used for the very helpful "Course Descriptions Report" that many colleges request from homeschooled kids). Then create one Lesson Plan for each course with a single assignment worth, say, 1000 points. Assign those to the calendar, mark complete, and give whatever points earned that will equal the grade your son actually received.
2) The other option is to print out OLLY's transcript report for whatever years your son is homeschooled, then use it as a guide to combine with previous school records to craft your own transcript in Word or Excel.
I'm sorry there is not a more straight-forward way to do this - we'll keep this in mind for future upgrades