Hi,
if it is only for FI then you can do it with a validation. else if this function has to work across all modules then you need to create a small ABAP development. I think this restrcition will be only till end of march. From April since you will again have the regular posting period coming in place, hence i feel you should not tamper with your FY variant (eg V3) or posting periods.
best regds
Subha