Anything can be done and of course your crazy your a grown man playing with trains.....
All kidding a side if your confident in your knowledge of decoder installation and your ability to work with brass engines the question is why wouldn't you put a decoder in it. I entrusted all my brass enignes to a guy who does it for a living and is darned good at it. People pay me for what I know and they don't so why should I be any different. I'm not going to learn on a $2500 enigne, even I'mnot that crazy....
Not really enough information to answer your question.
If you buy a brand new brass engine, it probably will come pre-decorated with a nice can motor and DCC/Sound already installed. If you're talking about a 20-30-40 year old shiny gold brass engine, you certainly could convert it to DCC but it wouldn't be a "drop in" installation. You'd probably have to insulate the motor from the frame and more-or-less rewire it. It's not particularly hard to do but will take a little time and effort...however if you get a good deal on a nice brass engine, it might be well worth it.
Here's a link to a pdf about installing DCC in brass locos.
http://markschutzer.com/Brass_Clinics/InstallingDecoders2008.pdf
Hope it helps.
Jim Murray The San Juan Southern RR
Is your layout DCC now?
Dave
Lackawanna Route of the Phoebe Snow
If I was to go crazy and buy myself a brass Camelback, can and or should I try to drop a decoder into it and switch it over to DCC?