In order to invoke a Process from within another Process one can use the "Start Process" Action available in the Process Designer's Actions pane. 

Parent to Child

We can transfer the value of a variable from the Parent Process to the Child Process, using the prefix "ext:" in the variable name in the Child Process.

For example, let's say that %NewVar% is a variable defined in the Parent Process. In the Child Process, it would look like %ext:NewVar% - same name + "ext:" prefix

Child to Parent

Additionally, the Parent Process can utilize the Child Process's variables' values, using the Global Variables concept available only in ProcessRobot.

Global Variables are variables that can be set and used from all the Robots and Processes. The Global Variables can be created through the "Settings > Global Variables" of Control Desk.

Post creation of Global Variables, using the "Get" and "Set" Global Variables Actions available in the Process Designer, one can retrieve and update the variables across the Processes.

Note that the "Start Process" Action works only when the Process is executed from the Production environment, like any other Trigger.

P.S We can also use the "Environment Variables" Actions to utilize variables across Processes.