It may not be possible to edit Gold Mines or the Gold Mine ability to provide lumber instead of gold.
Try copying a tree destructible, applying the gold mine model to it, and give it the maximum hit points; also add the Harvest (Ghoul's Lumber) ability to the Acolytes. Adjust each and every object editor field that causes conflicts.
This (potential) solution does not take into account haunting/summoning the "Lumber Mine." You may need to do some fancy trigger editing to make everything work properly.