Wanam Xposed Method
On a stock Samsung ROM, there is no option to edit the Multi-Window apps, and you can only select from a few apps.
However, you can work around this with an Xposed module called Wanam Xposed which lets you customize which apps are shown in that viewz as well as many other aspects of a stock Samsung ROM. To use this, you will first need root. There are many tutorials about how to root various devices on the XDA Forums. You will then to install the Xposed Installer and then activate the xposed framework and the module. You should then be able to customize many things on a stock Samsung ROM, including the Multi-Window apps.
Note: Please read all warnings pertaining to root, and research about root access before you try modifying your device, as it can lead to unwanted changes in your system. Always use caution when dealing with root or any modification of your device.