I like to use Superior drummer, but would like to replace the kick & snare with samples I downloaded off the net.
I cannot load samples from other manufactures into Superior, but I know people do it. How is it done? Do I need a separate trigger program?

Use another synth for the kick and snare samples such as Session Drummer. Copy the appropriate MIDI data to the Session Drummer track or create a custom drum map.

So do I put the samples I downloaded into the folder containing the samples for session drummer? Will session drummer see them? 

The samples don't need to be in a particular place but if you want to use multiple samples for a single instrument, it will require creating an sfz file. There are numerous examples of sfz files included with Session Drummer. The sfz files are plain text.  A simple file with samples for different velocities is "SL Groovy Kit Bass Drum.sfz" in the Session Drummer "Content\Kits\1 - Bass Drum". In this file the samples are in the "SL Groovy Kit Bass Drum" directory just below the sfz file location. Sample paths may be relative or absolute. I usually start with a copy of an sfz file and replace the sample names. While not absolutely necessary, by convention each instrument (kick, snare, etc.) have their own sfz files.

Superior drummer doesn't have what you need? What expansion(s) do you have for SD? Is it SD or SD2?
 
You can always load the EZdrummer kits too. They go on sale for good prices quite often. I have not needed another drumming tool since getting the SD2 and EZD expansion packs. I have a lot of samples too. Haven't used them in years.
 
Looks like you can add samples in the Session drummer kits library. Most are SFZ and FLAC, but you could probably put the WAV or other format in the libraries. I was able to load a sample for the instruments that way. Then you can save the kit too.

yep, velocities are something to consider for sure!

For quick and dirty triggering of a single sample (i.e. no velocity switching), you can simply drag and drop the sample on the desired kit piece "pad" in Session Drummer's Mixer page.