1. Its not really possible. A sealed system relies on the primary side (radiators, heating coil) to be pressured to around 1.3 bar.
2. With a vented (unsealed) system you would have a feed and expansion tank in the loft, say, with the boiler in the kitchen. With most houses having a ceiling height of around 2.4 metres, you have the F&E about 5 metres above the boiler. One (first) floor at 2.4 metres, say 1.5 metres from ceiling to boiler in kitchen, and perhaps 1 metre from F&E to first floor ceiling. This is roughly equivalent to a pressure of 0.5 bar, so not really enough to pressurise the system.
3. Going the other way round (vented to sealed) is much easier.