I found '...last but foremost...' with google in some texts, which seems to be some combination of 'last but not least' and 'first and foremost'. Seems to suggest that while something is mentioned last, it is yet the most important thing. Can it be used like this? does it sound right for a native speaker?
It sounds unusual – but not awkward. Because it's not commonly used, it could be a very effective way to stress the importance of a last point (say, near the end of a speech).
Moreover, "last but not least" infers the last item is no less significant than its predecessors, yet "last but foremost" implies that the last item is in fact more important than its predecessors, so the meanings aren't equivalent.
Speakers often begin with their main points, and then work "downward" toward more "supportive" material, which is why such phrases get inserted. The expressions are meant to convey, "even though I'm near the end of my presentation, I still feel this last point is important or noteworthy."
I agree with the other responses, it's not unintelligible or problematically awkward, but it is uncommon enough to be distracting in some contexts.
I would opt for "the final, and most important, point for consideration" or "the last item to discuss is also the most important."