It is not possible to create the Schema which is used for the Stub Components, but after your ECL provider is initialized and the Schema is created, you can modify the Schema and add metadata fields to it.
So the order of your process is important, but in the end it results in a customized Schema.
Mind you, your question does not provide enough details on what you are exactly trying to accomplish (for that I advise you to edit it and make this more clear, so answers can also be more detailed). Since you mention "...as one of its allowed type", it sounds like you want to create a Schema which would allow both ECL and other Multimedia Types. That is something which I don't think is supported in the ECL framework, as mentioned, you have the freedom to add metadata fields, that is about it.