Now I want to program a custom keyboard or I would like to edit the decimal pad. What is more easy?
The custom keyboard will be easier, since you can't access the decimal pad since it's private api.
If I would program my own custom keyboard, can I use the custom keyboard extension or do I have to create a view with a button for every number in a separate view?
I am not sure of your use of keyboard extension, I think your referring to an App Extension, in which case you should review the Custom Keyboard section. If you want to build a keyboard just for your app look at the 'Custom Views For Data Input' section of the Text Programming Guide for iOS.
Either way you will lave to create a button for each character your keyboard will shown.
Because I need the keyboard on many different views: Is there a possibility to create a view with the keyboard and to display the keyboard-view in a container view on top of the other views?
If you create a custom view for text input you would typically assign the custom view as the input view of the UItextField or UITextView your using. This will display your custom view where the keyboard would normally be. If you desire a different layout then you will have to handle the relationship between your text input view and your custom keyboard view.