No you cannot, BUT you can just fast-track best effort flows and only bypass fast-track for connections that need to be prioritized above or below best effort.
You can see the way my “Fast-Track Friendly QoS script” handles this:
http://forum.mikrotik.com/t/fasttrack-friendly-qos-script/102401/1
I get around the issue by only fast-tracking best effort packets; they can therefore be left as ‘no-mark’ and matched in that way. All packets that should be either above best effort or below best effort in terms of priority must be set to ‘accept’ before the fasttrack rule hits, and thereby those particular connections will not be fasttracked and can still be mangled and marked. That way you still get the benefit of fasttrack, especially if most of your traffic is best effort.