I have a hi-hat cymbal sound I like from the retro rock kit on garage band, but it decays too fast. Is there a way to make that particular sound last longer and leave the rest of the drums the same?
If the cymbal is synthesized, you should be able to adjust the ADSR and add a little S (sustain) to the note. But, AFAIK garageband doesn't let you mess with the tone generator on those internal midi patches. You can definitely do that with the instruments in Logic Ultrabeat or Drum Machine Designer. But don't go shelling out $200 for Logic until you try a little delay pedal action on the hi hat. Moving the hi-hat to its own track and applying one of the delay effect plug-ins might help.
If this is a Sample and not a MIDI synth, Your sampler should also be able to stretch the note a bit to add some sustain, however I don't know if Garageband does that.