By far the easiest way to do that would be like this:
var foos = fixture.CreateMany<Foo>(10).ToList();
foos.Take(5).ToList().ForEach(f => f.Bar = "Baz");
foos.Skip(5).Take(3).ToList().ForEach(f => f.Bar = "Qux");
foos.ForEach(f => f.Blub = 11);
Assigning values to properties is already built-in to C#, so instead of providing a restrictive API that can't possibly enable you to do everything you'd want to do, the AutoFixture philosophy is to use the language constructs already available.
The next philosophical step is that if you often need to do something like that, the SUT could probably benefit from a redesign.