The ‘Agent’ kits are somewhat fixed, however, you can mix and match them.
Method 1: (Great for CuBase Users)
Simply load different kits on a different MIDI channel. You can load up to 4 at a time. If you need more than 4, open a new instance of GA and keep building 
If you need all of your DAW track data for an extended multi channel kit to live on the same track, CuBase users can take advantage of the track drum mapping features and Diamond Drum editor to build it all quickly and easily on the fly.
Method 2: (I’ve only needed to resort to this if I’m using a different Host than CuBase, I.E. Sibelius/Finale)
If you need the kits to be mapped out on a single channel as if they are a single kit you can ‘remap’ an Agent Kit.
So, you would first load the Agent Kits you want individual instruments from and put them on the same channel.
Next, you would remap pads to the keys or MPC pads/banks you’d like playing them.
To remap Agent instruments you’ll first click “User Hardware Control Mapping” icon at the bottom GA4’s virtual MPC pad.
At this point, you can right click pads in GA4 and map them to be ‘triggered by’ any MIDI Note Names/Numbers you like.
You can have more than one trigger note on a pad if you like.
If you want to add custom samples, then you’d build a new User kit in one of your 4 Kit slots, add the instruments you want, and map them accordingly. Just leave pads and/or trigger keys ‘Empty’ that are already in use by one of the other 3 kit slots.
Alternatively, if you don’t feel like ‘remapping’ the trigger notes, you can stack kits and ‘mute’ unwanted pads. In a parallel kit…place whatever sound you want on the same pad location and make sure it is NOT muted.
Method 3: (To base entirely new and customized instruments/kits off of an Agent’s sample set)
Finally, if you happen to be working from inside CuBase 8 Pro or later…you can also instant render any samples you want from a Groove Agent Kit, and then use those samples in your own User built kits. If you want the samples ‘dry’, retuned, etc… Then go ahead and build an Agent preset that has all the individual pieces sounding ‘exactly’ as you’d like your RAW sample.
Next, simply build a little sequence on a MIDI track in CuBase that makes GA play all the kit parts you want one at a time (at different velocities if needed).
Finally, instant render that track, then go in and slice it up to make your own kit using those samples.
Happy Sequencing!
May the music be with you…