create a calculated column

Would like to know how to create a calculated column which is equal to other column. ie, i am having a column called BU in C-TYPE HUB and its propagated in my subscriber site collection say http://srvr1/sites/demo. i would like to create a new site column in Demo say BU_Calc that must be equal to the value of BU.i would like to use the BU_Calc for further processing