How to map two columns to one dimension with conversion file

Hello gurus, this is my situation:

SITUATION

I need to map accounts from a flat file to the model in SAP BPC.

1. Source (Flat File)

  Columns: C_Account, Cost Center, amount

2. Destination (Model SAP BPC)

   Columns: F_Account, amount

THE RULE

C_Account + Cost_Center = F_Account

EXAMPLE

Flat File                                                                 Model SAP BPC

C_Account  CostCenter                                             F_Account   

C_1982000, 2319  10$      must be mapped to -------->  F_3210000  

C_1982000, 2100  20$      must be mapped to -------->  F_3210000  

I dont know how to refer in the  conversion file both dimensions (C_Account, CostCenter) in the internal column in order to map correctly with an if statement.

Thanks for your help!!!!