Open up the instrument plug-in interface. It should be the EXS24 or, if you’re on 10.5, simply called Sampler.
In the EXS24 window (Bottom) there are two envelopes marked A D S R - these stand for Attack Decay Sustain and Release. The first envelope is assigned to the filter - no need to fool with this. The second one, on the right side, is assigned to the amp. As you increase the “R” (Release) the note will die out slower when played - more sustain !
Run your ARP effect as you adjust this to get sustain the way you want and not have the notes run together. Then save it with a unique name so you can easily find it for later use.
If you are on 10.5, the Sampler will have similar controls in there, but I didn’t stay on 10.5 long enough to work with the Sampler too much so I’m not sure to tell you exactly where the envelopes are - but they’re in there!
iMac 3.4 GHz i5, 24 gig ram, OS 10.14.6 (Mojave) RME FF800
Omnisphere, Trilian, RMX, Sup.Drummer