Is your requirement; do some business logic from WSO2 BPS and pass the output from BPS to your created proxy service in WSO2 ESB. Yes,this is possible to implement with WSO2 BPS.
You can follow up the references in [1,2,3] as the starting point on how to write a simple business logic inside a bpel and invoke an external web service with the logic output.
Please note,it will be easy for you,if you can start developing bpels with WSO2 Dev Studio eclipse plugin[4] as it provides GUI support to create the business process.
[1] http://wso2.com/library/articles/writing-simple-ws-bpel-process-wso2-bps-apache-ode/
[2] http://thiliniishaka.blogspot.com/2012/10/develop-ws-bpel-process-using-wso2.html
[3] http://thiliniishaka.blogspot.com/2012/10/part-2-developing-ws-bpel-process-using.html
[4] http://wso2.com/library/articles/2012/09/develop-wsbpel-apache-ode-wso2-developer-studio/
Thanks;