Since no one else has responded yet, I'll see if I can help, but I don't consider myself an expert.
From what I've read online, the RD300NX is GM2 compatible. If you go into Preferences>Midi>Instruments and assign an instrument type of GM Level2 (or GM Level2 Drums) to the device's channels, it will allow Sonar to access all of the sounds that the internal tone generator is capable of. I'm not sure if that is what you're after.
If you want to go through a little more work, you can "import" the RD-300 section from within Cakewalk's Roland.ins file using the Define...>Import... functionality and browsing to your Cakewalk Content\SONAR Platinum (or whatever version you have)\Instruments folder. That may give the same results, or it may be a little better or worse, depending on what you are looking for. I can't say for sure where your Cakewalk Content folder is located, since that is user configurable. The default location is C:\Cakewalk Content.