While it's technically possible do do that using our NonPublic API, might I suggest a different approach?
By default, every fake type that is created using Isolator's AAA api is "recursive", meaning, all methods and properties are fakes. You can use the following to have the fake object call an original method:
Isolate.WhenCalled(() => fake.GetSomething()).CallOriginal()
In your case, if your SUT (Foo) has a dependency on Bar, I suggest faking it using the following API:
Isolate.Swap.NextInstance<Bar>().WithRecursiveFake();
var sut = new Foo(); // all calls to Bar are now faked
...
or if you need to set a certain behavior on Bar:
var fakeBar = Isolate.Fake.Instance<Bar>();
Isolate.WhenCalled(() => fakeBar.IsBaz).WillReturn(true);
Isolate.Swap.NextInstance<Bar>().With(fakeBar);
var sut = new Foo();
...