外部软件包的自定义golang JSON封送处理

How can I add custom JSON marshaling to structs from external packages? I am unable to add the MarshalJSON and UnmarshalYAML to these structs.

You could add that struct to one defined by you. Then add the functions MarshalJSON and UnmarshalYAML to your struct:

type YourStruct struct {
    ImportedStruct
}

func (s *YourStruct) MarshalJSON() ([]byte, error) {
    ...
}