Is it possible to combine them like
* html @import url(ie6hacks.css);
No. at-rules like @import are not selectors, so cannot be combined with other selectors.
There are ways to make at-rules work as hacks, for example this:
@import url(/* no! */iehacks.css);
will be loaded by IE6/7 but not the other browsers. However, I wouldn't recommend using it; this sort of thing can be really fragile. This particular example is also invalid CSS.
As Daniel says, if you want separate .css files for hacks, the best approach is a conditionally-included link tag. The beauty of “* html” is that you can put hack-rules in the same stylesheet, which is easier to manage if there are only a few of them; if you're having a separate style sheet anyway, it offers no advantage.
IMO “* html” for IE6 is the only hack it's still legitimate to use today. All the box model stuff is dead along with IE5 — assuming you're not using IE6 Quirks Mode, which you shouldn't — and the other browsers, even IE7, are generally too good to be able to attack with a simple hack; the few hacks that can target them are too complex/fragile/invalid to really use.
(And as the inventor of the Simplified Box Model Hack, I say a hearty good riddance to them.)