How can I install a portable application that is not in your suites or your catalog, but I have downloaded from another source?
That is: How can I make it appear in the Liber Key menu?

It's very easy.
In the directory where LiberKey is installed you will find a folder named "MyApps". It is recommended to copy any external portable application in this folder. Then drag&drop the executable file into the LiberKey menu in order to create a shortcut in the menu. That's it!

Thanks!
And how do I remove a shortcut without uninstalling the application?
How do I add a shortcut to an own application under an existing group in the main menu (red circle with green spot)?
How do I add a new group to the main menu?