How to add custom ROM chip to Proteus?
Unfortunately no, it is not possible. The only way you will be able to do it, is to ask Proteus to add it to their library via the library and model request forums (only available to view by those signed up to the forums via a valid user license), or to create the part yourself. 

This document should be a good starting point. There is a lot to take in to fully create a part you can use in VSM simulation. 

There is also a shortcut way. Once you create a new part, you can right click, go to properties and tick the "Attach hierarchy module" box. Once you have done that, you can right click the component again and select "Open child sheet". This will allow you to model the internal circuit of the IC, or you can insert an IC with the same specs and functionality, and use the terminals mode to link the pins to your created model.

The only issue with this though, is you can only use it on that particular design, you will be unable to use it in any new designs.