Yes you can!
Simply provide the mixin during a reopenClass invocation:
// The mixin itself
FooMixin = Em.Mixin.create({
...
});
// Mix in at the instance level
BarClass = Em.Object.extend(FooMixin, {
...
});
// Mix in at the class level
BarClass.reopenClass(FooMixin, {
...
});
I stumbled across this problem as well, and discovered this being done in the Discourse project.
Hope this helps!