Allocation of cost from one business area to another business area

Hello friends,

Client Requirement: They have 5 business areas. one is for Head office and other 4 are business operations. Their head office expenses are booked under various cost centers in H.O Business Area. Now they want to allocate all H.O expenses to other business areas where business operations are carried on.

currently all allocations are made manually. now they want to incorporate in SAP.

is it possible to allocate the expenses which are booked under one business area to other business area???

pls give your ideas for how to allocate those expenses.....