Yes, you can define an "App" to include as many Process Models as you want.
Within the "App Designer" select the "Apps" tab and then select "Create App" highlighted in the red boxes below:
You will then be given the option to enter an App definition name (mandatory) and a Description (optional). Here you can see I've named my App, "NewApp".
You will then see the following form where you will want to select "Edit included models" which will then present you with a list to select from all your existing Process Models:
You can see in the following image where I've selected the first 3 processes to be included in the "App". Each of these now has a plus sign in a blue box displayed in the upper right corner of each of the selected thumbnails of my processes. These can freely be selected and deselected as you determine which processes to include in your App.
Then all you need to do is close and publish the App. If any of the selected Process Models are already being used in another App you will get a friendly warning to let you know after which you can Confirm and close the window. In my example below each of the 3 selected processes had previously been included in other Apps so you see the warning.
Now when you go to the Process Workspace and start my "NewApp" (in the bottom right corner), you will see the option to start any one of the 3 processes that I had included in the App.
Please let me know if that helps answer your question or if you have any other questions.