What are the differences between chaincode (in golang) and the bna file (created from hyperledger-composer)?.
And, how to add a peer into an existing hyperledger fabric network?.
This article can explain as a side-by-side (Golang and Composer model-driven approach) comparison -> https://blog.selman.org/2017/07/08/getting-started-with-blockchain-development/. The main difference is Hyperledger Composer is a set of abstractions, tools and APIs to model, build, integrate and deploy a blockchain solution (eg a business network for parties to transact on) to the blockchain network. Composer business network archives may be deployed to Hyperledger Fabric for execution.
As for your Fabric question - in order to add new peers to an existing organization you need to generate new certificates for your peers and sign them using the key of the root CA certs for the existing organizations. See this SO and links therein for more info -> Add Org or peer in Org dynamically in Hyperledger fabric