The thing is that you cant open the fiscal year. Once it is closed, its closed forever unless you customize. So you can't create depreciation entries with back dates, but I advise to do this: Make the value of the purchase asset equal to the net value which would be at the start of open year. You will loose the old(closed) year depreciation expenses, but you will have the correct asset value and depreciation entries for the current(or opened) years.
Like: Closed Years are: 2013, 2014. Opened is: 2015 Asset value at 01.01.2013 was 100 000. Accumulated depreciation for 2013 and 2014 = 70 000. Then take 30 000 as the purchase value at 01.01.2015 and go on.
This advice lacks the information about old year depreciation and initial asset purchase value, but once years are closed, they are audited, so no worries:)
Thanks #Zura I don't have enough Karma to comment on your answer That's exactly what I was thinking to do, and I will mention the original cost for each asset in an additional field as just an info.