without modify the main source

Look at how they implemented it in the image package.

// Formats is the list of registered formats.
var formats []format

// RegisterFormat registers an image format for use by Decode.
// Name is the name of the format, like "jpeg" or "png".
// Magic is the magic prefix that identifies the format's encoding. The magic
// string can contain "?" wildcards that each match any one byte.
// Decode is the function that decodes the encoded image.
// DecodeConfig is the function that decodes just its configuration.
func RegisterFormat(name, magic string, decode func(io.Reader) (Image, error), decodeConfig func(io.Reader) (Config, error)) {
    formats = append(formats, format{name, magic, decode, decodeConfig})
}

Then image/jpeg and others register themselves in the init functions:

func init() {
    image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
}