As far as trenches are concerned, you can now! On June 1, 2002, master addon maker, GFX707, of OFP Fortress fame (the site no longer exists), came out with his Bunker Pack 3. You should be able to find this and other GFX707/OFP Fortress addons at the numerous OFP addon sites out there on the WEB.
Regarding, tunnels, master OFP addon maker Kegetys is working on it but he says the textures aren't good enough yet to be released.
