In Sage 50, there is no way to avoid closing the previous year when opening a new future year.

Opening 2018 will close 2016.  There is no way to re-open it or undo the change, and no facility within the software to 'merge' two sets of books (i.e. a 2015-2016 set into a 2017-2018 set) or to import older entries.

There may be other software packages that allow doing that, but in Sage 50 it's best to get a working system up and running by opening the current and one year previous, then make entries to as of the earliest open date, to adjust beginning balances as new information is uncovered from the shoe-box.

Dangit.  Thanks.