Copying content from an existing game.

Hello folks!
My question is very simple. If I have an oldish (2000-2003) game for pc, can I access its maps and textures and change on ureal engine 4? Once I do that can I resource back the game and play it with the new material?
I have a game that has only cmap files (i guess they didn’t have fbx at the time). Anyways, this is only for fun and experimenting. Thank you!