Are you having an issue with trying to mine a cave wall? If so it is due to the server and client not having the same patch. I had the same problem when I was running lower client version than the server's.
If however you want to add new static items that you want to mine from, you will need to add the ItemID to the m_MountainAndCaveTiles list. Either at the beginning or end , doesn't matter.
If you want to have different resources, however on your specified item you will need to create a new HarvestDefinition yournew;
then yournew.Tiles will need to be assigned to a int[] containing your itemid.
Hope i've made it clear enough. I'm Pressed for time.