Hi Seb,
If you are using an analog sound, it means it is created with a synthesizer, and not with a sample. Therefore, just as you won't find a reverse option in any virtual analog synthesizer, you won't have this feature in Spark.
It is possible to import individual samples to pads, you can drag and drop them into pads. So one solution would be to render your sound to audio, put it back on a Spark pad, and reverse it.
I took a look at the 909 kits (DR-909 and dirty-909), and neither of these have an analog cymbal, they are all samples.
Therefore I don't know which instrument you were trying to use, but take a look at the 6 instrument parameters in the Studio panel (bottom panel), maybe the sound you use has both Attack and Decay as parameters. You could get a reverse effect with an analog sound by using a long attack and short decay (invert the two values actually)
That could be a nice solution, and you would be able to change the length of the reversed cymbal by tweaking the attack parameter.
Best,
Seb.