How to extend a basic item like inserter, transport belt

[Epix]

For example, i want to make an inserter that can pick up items from two position, or a transport belt that have one line or three line.
First thinking comes to me is, extend the inserter and modify its pickup method. However, I don't know how to extend the inserter nor which method to modify.
Any help?
I'm a newbie to lua and factorio modding. Please tell me if I should do it by other method like event or something. I may not have a thorough knowledge of game mechanism...