you can't really do it in the Brera/Spider/159 without major expense or hassle. aren't the main globes in your car 'dual filament'? that is, it serves the purpose of both the lowbeam and highbeam (the centre-most lights in each cluster are the highbeam 'flasher', but not the main highbeam).
factory xenons are a 'bi-xenon' design. it has a single filament HID globe, but the housing has a moving 'shutter'. when you switch to high beam, the shutter moves up and out of the way, 'revealing' full beam.
you cannot simulate this with an HID conversion kit in your car, unless you convert to bi-xenon (remove existing lens and custom-graft in a bi-xenon projector lens; or replace entire light assembly with OE xenon versions...$$$).
the most practical upgrade is better quality globes, and if you particularly want the colour of xenons, then something like Philips Crystal Vision or Diamond Vision, or PIAA 5000K.
having said this, a mate of mine has actually fitted an HID conversion kit to his 2.2 Brera. i must say, i haven't asked him about the highbeam situation.