That depends entirely on what the ERP is & how it is architected and what your add-on module is & how it is architected. In particular it would depend quite a lot on the integration strategies choosen and supported by the two parties.
App-servers may, or may not, have a role to play.
Perhaps you could provide an example of something that you actually want to do? If you keep it abstract then all answers are possible but no answer is correct. Furthermore it would take months just to cover the high level possibilities without ever getting into anything specific enough to act on.