Call Transaction Code from WebDynpro JAVA

Hi,

I want to call a Transaction Code from from R/3 thru WebDynpro JAVA, and I also need to pass some parameters to the screen.

My scenario is as follows: I have a Webdynpro screen which gets user info (step 1) and once the end user clicks SUBMIT, it will call a T-Code from R/3 (Step 2) by passing parameters from step 1 and I also want to show the T-Code screen with a custom button to navigate to another WebDynpro screen which would be (step 3). So, it would be a round trip where I want to communicate with Webdynpro and R/3 alternatively.

Guide me how it would be achieved thru WebDynpro JAVA...

Regards / Guru