In short, no you can't.
The 1.8T engine uses a recirculating boost system (as with all modern VAG turbo/supercharged engines) so the excess charged air from the turbo is fed back into the system when you let off the throttle, rather than being vented to atmosphere causing the "pshhht" noise associated with blow off valves. Your ECU is always monitoring the air going in to the engine, any abnormalities in these measurements can cause running issues for your car.
Fitting an open cone type air filter/induction kit will help produce a type of dump valve/blow off valve noise when you change gear which is usually mostly inaudible as the sound gets absorbed in the factory air box.
Here at GolfGTI.co.uk we would not recommend anything other than a fully recirculating valve, even if your mate down the pub has had a blow off valve fitted for years with allegedly "no issues"
If you have the standard plastic diverter valve fitted but are getting a strange wastegate type chatter (or pigeon sneeze as I call it) then your diverter valve is most likely broken and can be replaced with a new part from the dealers for around £40. Currently the part number is 06A 145 710P and dealership parts departments will know it as a "cut-off valve". There are aftermarket valves available, some are good some are not so good but they are all generally 2 - 3 times the price of the standard item which works very well in it's latest revision. The aftermarket ones tend to be shiny though if bling is your thing.
Hope that clears up some of the myths and reduces the amount of times we get asked this question