modify display list

I have created a display list. How can I add more entries later to the list?

I dont want to create a new one.

Yeah, that’s an easy one: You can’t!
Display lists are defined between glBeginList and glEndList and that moves all the data from client to server space and there is no interface to add something to an existing display list.
You can only regenerate all the stuff without getting a new ID but overwriting with the same display list ID.
Another method would be to generate an array of lists and for all add-on data you build a new list and call the whole bunch with one glCallLists and an array of indices.
However for dynamic data immediate mode and/or vertex arrays would perhaps be more convenient.