Evnabby : "How can I get them into the drum machine so I can trigger them?"
Not easily :-)
The program (.prog) file points to the various instruments.
This file is located in C:\ProgramData\Cakewalk\Studio Instruments\SI-Drum Kit\Programs
The prog file is uneditable, but readable with Notepad. In it you'll see pointers to various instrument definition files (.sfz).
These files are located at C:\Program Files (x86)\Cakewalk\Studio Instruments\SI-Drum Kit\Kits
The sfz file is editable with Notepad. In it, you'll see pointers to the WAV file samples.
Normally, you could either change a file path\name to another one or replace the wav file pointed to with another with the same name. However, these SI-Drums sfz files contain a couple of (undocumented ?) parameters that may or may not prevent any of this from happening :
encryption_scheme=1
encryption_key=f9124d38933b6a
I have no idea what these parameters do.
You can get the sfz specification off the cakewalk.com website, but it doesn't mention encryption.
To use mp3 files, you'd have to first convert them to wav files, but they probably won't sound good.