Althoug I have not tested it, I suggest

To enter the assets  and corresponding accumulated depreciation via the accounting module (using the Miscelaneous journal for instance or a specific one that you will be able to create (for instance Fixed assets journal). 

Debit Assets accounts/ Credit Depreciation for each line of assets.

The global accounting counterpart for the above entries will be a balance sheet account (credit) representing prior years of accumulated earnings or losses.

Normally, the accounting for existing assets and liabilities  (whether fixed ones or inventory or cash or debts) is done as an opening accounting process where all debits and credits are balancing. This is the opening balance.

Then Odoo should be able to resume the depreciation schedule for the remaining net book value and the remaining time life of each assets, based on the depreciation plan to be created.