Here, try this:
GE 0127 Planet KARMA 2 - this has a Velocity Envelope assigned, and several GE RTP that allow you to control aspects of the envelope.
I'm going to discuss doing this in Program Mode with program GM 048 Timpani. So select that program, and turn on KARMA. Leave LATCH off for now.
Select GE 0127 "Planet KARMA 2" on the KARMA GE tab.
Play a note (such as G2) on the keyboard: pretty cosmic!

But let's get back to tympani rolls.
First off, go to the KARMA GE Page, and set the following RT Controls as specified:
Slider 5: Note Range = 10 (limit to playing two notes alternating)
Slider 7: Repeat Repetitions = 0 (turn off repeats, which by default is on max)
Now, when you play a note on the keyboard, you should get a two note trill that fades in and then somewhat out, down to a certain low level. That's because, when loaded, the Velocity Envelope looks like this:
The red-circled parameters are the GE RTP that have been provided in this GE to vary the control of the envelope (and hence are grayed out in the KK software).
You probably don't want the tympani roll panning left to right, so go to the KARMA > MIDI Filter tab, and turn off "Transmit MIDI Filter: CC-A".
Go to the KARMA > GE RTP > Master A > 17-32 page. (If doing this in a combi, you would want the Module A tab.) You will see these four envelope-related parameters:
25 Env: Env On/Off [1] VE
26 Env: Env Start Level [1] VE
27 Env: Env Sustain Lvl [1] VE
28 Env: Env Time Scale [1] VE
The "[1] VE" at the end of each of these indicates that we are talking about Envelope #1, and the "Type" is VE Velocity.
If you want to control the amount of time the fade-in/fade-out takes, you vary #28 Env: Time Scale. A value of 1 would be extremely fast, while longer values will increase the amount of time by varying degrees. Try several different values and listen to the results.
As shown in this second image, if you don't want the roll to die back down (fade out), you raise GE RTP #27 Env: Sustain Level up to 99, and it will stay at the high level. You can also adjust the level that the envelope starts at (by default it is 26), but set #26 Env: Env Start Level to 0 to fade-in from nothing.
Finally, if you want the roll to die out nicely when you release the key, turn on the LATCH button. This enables the release portion of the envelope.
Have fun!