Hi Amol,
Unfortunately, I don't know of an easy way to mix and match process flows from different projects. That doesn't mean it can't be done, I just haven't seen it before! The obvious trick to get some process flows from one project into a new one is to do a Save As, then in the new project delete the process flows you don't need. That would not accomplish what you are suggesting though!
The only solution I can think of is to take the code from the existing projects and mix and match them into code nodes in a new project. Remember you can do File / Export / Export All Code to get all code from a project.
If anyone has better ideas, I'm all ears!