Pretty sure it's not possible to add return springs to the Perlick or any other FC faucet because of the way the flow control shuttle works.
I've got four Perlick 630SS flow control faucets and really like them, in addition to two old Shirron Micro-Matic forward seal faucets, and recently acquired an Intertap Nukatap. I'd pretty much used the Perlicks exclusively for the past 4-5 years, but have begun having problems with the front seals properly closing. I'd never disassembled them or replaced parts, so during the rebuild process I bought a new Nukatap to stand in while the Perlicks were out of service for repairs.
I wasn't expecting the Nukatap to be anything but a stand-in, but WOW, was I impressed. Even without flow-control, side-by-side with a rebuilt Perlick FC the Nukatap gives a much smoother, foam free pour. The as yet unanswered question is how durable the internal mechanism will be over time and a few thousand pours. I fought foamy pours for a long time with standard rear sealing faucets and the forward sealing Shirrons, only finding consistent pours when I went to the Perlick FCs. But this Nukatap is really delivering the goods. I'm strongly tempted to try one of the FC Nukataps when they finally hit the market (it was supposed to have already happened), but honestly I don't think I need it based on the performance I'm getting (so far) from this non-FC Nukatap.