yes you should be able to edit and change submodels you have. If you are working with Simcenter Amesim 16 this is under Submodel Editor. If you are using an older version of Amesim you might find it under LMS Ameset. In this tool you have access to the equations ruling a submodel and can change them if necessary. Prerequisite to be able to do this is to have the C-Code available and this will depend on your installation (I recommend discussing this with your local support team. They will be able to grant you access to the code if not available and if possible).
In general we recommend using this feature with caution since all our submodels have been validated over the years jointly with partners, customers and also from our own work. If you feel one of our submodels is not enough for the purpose of your work talk with us and we will be glad to help you. Many times what you are looking for is just available in a different submodel. I am sure one of our experts will take a look at your request and will let you know what is possible and what is not.
I am not sure I understand your question. I am also not an expert on the topic...at least no longer as I have moved to a different part our structure some years ago so some things are no longer the same.
Let me invite a Siemens Phenom @Emmanuel_D to help in this discussion. Maybe he can give you some directions.