How to convert a .pkg into an .app
A .pkg file is basically a folder (if you will) that the installer "app" is located in. I think you are wanting to burn/run the installer from a disc/USB? (if not, you can run it from the .pkg file itself by double-clicking and mounting the installer. 

That being said, this is how to locate and save the .dmg file locally.

What you want to do is:


Right-click on the .pkg file > select "Show Package Contents"
Open the Contents/SharedSupport folder and locate the file named
InstallESD.dmg.


Copy InstallESD.dmg to another directory of your choosing. This InstallESD.dmg file is what you need to burn/run macOS installer from a DVD?USB. 

Make the OS X El Capitan Bootable Installer Drive with a Terminal Command



Launch the Terminal application, found in /Applications/Utilities/
and enter the following command exactly (unless you changed the
target volume name from ElCapInstaller to something else) onto a
single line, the text will wrap because it is long, but it’s
important to have proper syntax:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/ElCapInstaller --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction

Hit the Return key and enter the administrator password when
requested.



  You’ll then see the following screen text in the Terminal:
  
  Erasing Disk: 0%… 10%… 20%… 30%…100%…
  
  Copying installer files to disk…
  
  Copy complete.
  
  Making disk bootable…
  
  Copying boot files…
  
  Copy complete.
  
  Done.


The target USB disk or flash drive will be erased first and then copy the files to it so that it will become a bootable OS X El Capitan installer. This can take a while to complete, so wait for the “Done” message before continuing.

When you see “Done”, that’s it, your OS X El Capitan installer drive has been created, it’s bootable, and you can use it to update as many Macs as you want to with OS X 10.11.

To boot from the installer drive, hold down the Option key during Mac system start, and select it from the startup volume menu.



Note: the OP is trying to create a bootable disc via MAS and Terminal method. In the instructions it tells you to download and save the installer, to avoid deletion after install completes and saving it for future use. It goes on to say load the .app to continue with creating the bootable disc/usb. This is why, I assume the user has worded his question as such.

I wish people would actually wait for the OP response without thumbing down prior to knowing, if indeed the answer is right or wrong. Or maybe explaining why the answer is wrong by submitting an answer or correcting a wrong one.