Historical Closed - Need to enter AR

I'm new to Sage and Bookkeeping.  I have a client that is so disorganized I could cry.  I just got this information regarding outstanding balance from my client.

Because I am not able to get a clear understanding of which clients have outstanding balances as of the first of the fiscal year, I wanted to put in the lump sum under one customer.  I guess at some point i closed historical so that screwed me up. I can't create a new customer an pop in an opening balance either.  How do I do this?

I don't want to enter in another invoice cause the balance was claimed on last year taxes and that will also mess up the taxes.

I need step by step instructions here.  Like I said I'm new.

Please help.