I'm afraid Palace is right. Your sound will never be better than the weakest link in your signalchain.
However you can improve the sustain a little by lowering you pickups if they're close to the strings. The magnetics in the pu's will "pull" the strings so they stop swinging earlier than they could. Anyway if the wood in the body isn't relatively resanonant and the pickups are of poor quality, it probably wont help much. Still worth a try. Also make sure the action isn't too low. Too much buzz will kill your tone.
As for the Fernandes sustainer, it's said to be quite expensive, and also pretty difficult mounting. Probably not worth it.
There are a lot of sustainer/compressor pedals on the market, but again, a pedal like these won't help much if the the rest of your gear is of less good or poor quality.
Whick amp do you have, by the way?
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