Triggering BPM process based on a correlation

Hi

I need to start a BPM based on a correlation. But the actual response of the correlation is not feeding to the subsequent mapping. Can i acheive this?

I have the scenario as Message A coming into BPM and message A is mapped to Message B in the transformation step. But this transformation should take place after the correlation of Message A with Message C using an ID as coorelation ID.