hotchocolate schema stitching with another language server

I have a question for a custom scalar converter and schema stitching. I want to stitch a schema from a third party Server and from my own server.
But the other server uses Java and a "BigDecimal" Type.
And when I load the schema in "banana cake pop" or any other UI tool (postman, insomnia) I get the following error on the server:

"1. Unable to resolve type reference BigDecimal. (HotChocolate.Types.ObjectType)".

I have written my own Scalar (like so, or so) BigDecimal type and it is visible in the schema but it isn’t used for the BigDecimal Scalar from the thirdparty server.

Is there a way to allow or overwrite thirdparty Scalars so I can use them in .net?
Is it even possible to have custom types which comes from other graphql services? I couldn’t find anything to it on the internet.

Thanks and Best Regards in advance