Hi,
Instead of getting this in BEx. You can achieve this in Cube/DSO level(assume that this report is on cube). Create a new year infiobject. In transformation map the posting period with year by taking rule type as forlmula, inside formula there are date functions available, try with FISCPER_FISCYEAR date function. You can try this in Dev system and see if this gives the expected result.
Regards,
Vengal.