I am planning to get a Athearn MP15AC with sound soon. Is it possible to add ditch lights to it with the decoder that comes with it?
Most likely, no. As a general rule, decoders that manufacturers put in their locos are cheaper ones that have minimal functions. To really find out, you will need to contact the manufacturer and find out the specifics of the decoder sold and installed in the unit.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Here's the SoundTraxx decoder CV listing for the Athearn MP-15 locomotive from the SoundTraxx web site.
http://www.soundtraxx.com/factory/OEM_pages/athearn/mp15_ho.pdf
CVs 51, 52, 59 and 60 would adjust the functioning of ditch lights, but whether the decoder has provision for connecting ditch lights is something you would have to ask the people at SoundTraxx.
Don't bother asking Athearn -- they won't know.
Highly unlikely that the decoder would have provisions for ditch lights. The decoder used in the MP15 is a hybrid and not available separately. It's also a v-e-r-y tight fit. Not a particularly fun install.