I remember you had exact question some time back also and that time we provided with the solution.
Let me give you a live example:
In one of project, my client fiscal 2012 ended on 12/31 and they are already posting transaction in 2013.
Also, they are making back dated transaction in 2012, mainly adjustment entries, bank charges etc. They will be closing Fiscal 2012 in AX only when their accounts are audited and that will not be before time around Jan End or even later.
After that they can close 2012.
This is what happens practically. No one closes year on 12/31. You can go ahead with this. Create year 2012 and 2013, post the opening in 2012, close the year and continue transaction in 2013.