Can I add new function for protobuf generated struct

Question

I am using protobuf definitions to define message types. Structure looks like

message1 {
}
message2 {
    bytes msg1 = 1
}

So message2 has a field which is a byte array representation of message1. I am:

1. Creating a msg1 struct instance
2. Serializing it
3. Encrypting byte array obtained in step2
4. Compute digital signature on cipher generated in step3

I want to see if it is possible to add a function for protobuf message1 that will return me a serialized message1.