Correct, Dahan are not limited to the starting pieces, although there are not many ways to increase their numbers. Other powers that do are River's Bounty (River Surges unique) and Cycles of Time and Tide (from B&C). There may be events that add Dahan, but I can't search that now.
There are four events that can add one Dahan (per board) to specific land types if there are already Dahan there. During one game last week my friend and I actually hit all four of them in a row.