Hi Venkat
You can import both BAPIs in PI and call from an external system. In this case you would need a RFC type T. You will define a program id in SAP PI and the external system will register in SAP PI.
You will be able to transform the BAPIs in PI to a web service. You would import the BAPIs too.
Other alternative is using IDOCs COSFET for requesting coste centres or COSMAS and PCRMAS through SAP PI. It will depends of the kinf of cuntionality you need.
It is posible that in the ESR you find a web service that call the BAPI but this kind of integration should be imported in PI too usually.
Kr