Welcome, John.
To answer your question, for the most part, yest there is.
Is there a hardware box I can plug into the USB port ....
Yes. There are dozens of them available. You are looking for a USB Audio/MIDI Interface.
These come in several configurations. If your controller has only MIDI(DIN) out, you will need an interface with a standard MIDI input. Pricing will vary depending on number and type of inputs, and overall quality.
Check here to get an idea of what's available:
http://www.sweetwater.com/c695--USB_Audio_Interfaces/pn2 ...that would have a MIDI synth that I could trigger with my keyboard?
No. But that's actually not a problem. The Santa Cruz was a Soundblaster clone, and as such, had an onboard synth.
None of the current interfaces have built-in synths, but they are not needed.
VST synths work within the Sonar framework, and offer much more flexibility.
All versions of Sonar X3(the current version) include some VST synths. You can also use VSTs from other vendors.
This give you huge options, and just about any sound you can imagine is at your fingertips.