Just to confirm, YES you can chnge the main beam bulbs easy enough but they will NOT be up to the performance of your factory fit Xenon HID's!
The best I could suggest is a bulb with a colour match as close to the original dipped beam xenons as possible. The ones from the bay that claim to be xenon 'filled' won't match the quality of your dipped beam technology because that invloves a specialist 'ballast' and starter pack with high voltages invloved. The main beam are simple 12V.
An additional retro fit HID system will not work either because as you know, that system takes a couple of seconds to 'settle' - no good for flashing main beam lights!
The advice, don't buy cheap. If you want quality and performance be prepared to spend £30 +. I would recommend some Philips 'X-Treme' power +80% or some PIAA 'Extreme White Plus'. The fitment you require is H7.
Interestingly, my mk4 astra uses H7 for dipped beam! :-?