Sub process association

We have a parent journey which has a sub-process attached to it. That sub-process get invoked through two ways.

  1. One through normal parent process.
  2. One as independent process with parent process id as process instance variable. This can be called as many times as we want. Not a fixed value.

How can we associate the sub-process initialised through step 2 to the parent journey.

Please help !!