hi pavan,
no need to assign the customer to another sales area you just extend that customer into different sales area its possible in sap
go for the XD01 and enter the customer number and maintain the sales area which sales area you want to extend this customer you have to maintain and below you can notify the reference column here you need to enter the same customer as well as existing sales area..then system is going to be extended the customer into another sales area ...
you will try in IDES system...
hope it would be clear for you..
regards,
rayudu