With the DA15 Limited, you can use a Lee or Cokin adapter if you put a regular 49mm filter on the lens first. I tested with a Cokin adapter because I don't have a Lee adapter that small. With the 49mm filter and Cokin adapter, there was very slight vignetting. The 49mm filter sticks out slightly from the front of the lens's hood. If you were to use a "slim" 49mm filter with external threads (some become "slim" by omitting the external threads) and the Lee "wide angle" adapter, you may be able to avoid the vignetting entirely.
Ideally, you would remove the glass from the filter and just use its threaded ring as an extension adapter.
Formatt Hitech 49mm Firecrest SuperSlim UV MC Filter FC49SMUVMC
---------- Post added 07-12-15 at 10:21 ----------
Testing with the DA21 Limited and a Cokin adapter, I saw no vignetting at all.