Ores in Minecraft Server?

I made a minecraft server a while ago and i have a flat map (which i want). The map has no mountains or hills or bodies of water which is great for my need. Only thing is there are no ores underground so nobody can go mining. All you have is dirt and eventually bedrock. Is there ANY way that i can have a flat map and still have the ability to go mining and find coal, gold, diamonds, etc. I really want the map to have nothing on top or the ground but i still want people to have the ability to go mining. Can anyone help me? Thanks :D