The previous advice is good -definitely 'pre-tin' the brass parts first and then allow the solder attached to the brass to flow on to the white metal which is pre-fluxed.
A good trick to avoid having to use too much heat on the brass is too grind away any large contact areas on the brass eg. leave a thin outer wall on the brass - helps quicker heat transfer and less chance of destroying the white metal.
I use Powerflow Flux and low heat lead free solder. You could also use a heat sink material on the white metal to stop too much heat transfer - have a look for it at jeweller product websites.
http://www.fernox.com/?cccpage=powerflow_flux&sub=9
At best, soldering white metal even on it's own is difficult and needs lots of practice. Here's a piece I made from soldered sheet pewter...drove me mad trying to get it right!
