Clr-Namespace URI's without an assembly mentioned only work as compiled Xaml.
But in case you provide assembly with namespaces like - ";assembly=AssemblyName"
suffix to your Uri, it will be parse successfully.
But in your case xmlns:Custom="http://myCompany.de"
, you can't add assembly reference. so, i think the only way out is to add the mapping manually for this case.
And for custom namespaces belonging to your project, you can use the assembly
suffix to get them parse.