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 think that "last but foremost" is actually somewhat less intuitively logical than its true converse, "First and least," a phrase that makes perfect sense in situations where the speaker or writer is presenting information in ascending order from least to greatest importance. Understanding "last and foremost" to mean "most important even though last" doesn't take superhuman effort, by any means, but I think that a better way to express that idea is to say "last and most important" or "last but most important."
From the perspective of a reader or hearer, the trouble with "last but foremost" is related to the fact that foremost as an adverb has not only the meaning "most importantly" but the equally strong meaning (according to Merriam-Webster's Eleventh Collegiate Dictionary) "in the first place"—that is, in the place opposite the one where the item appears in a "last but foremost" list. Likewise, foremost as an adjective has both the meaning "first in a series or progression" and the meaning "of first rank or position: PREEMINENT."
It's not that an audience can't figure out the intended sense of "last but foremost"; it's that the effort involved in reaching that intended meaning is avoidably large in comparison with the effort required to understand "last and most important," "last but most important," or "most important."
That difference in effort may explain the limited popularity of "last but foremost" in Google Books search results: A search for occurrences of that expression (the blue line on the chart)—along with the expressions "last and foremost" (the red line). "last but most important" (the green line), and "last and most important" (the yellow line)—in the Google Books database for works published between 1800 and 2008 yields this Ngram chart:
As the chart indicates, search results for "last but most important" are somewhat more common overall than matches for either "last but foremost" or "last and foremost"—and results for "last and most important" are far more common. The "last/most important" combination thus appears to be to be overwhelmingly more popular than the "last/foremost" combination in published writing as a way of expressing the idea "last in order of presentation, but greatest in significance."