Remember who it was, by the way? What was the story of the woman who became a pillar of salt? Well, I trust that this answer you can find on your own, and I'll help you to express the "became" word. In this case what we need is - the verb לַהֲפֹוךְ! Why am I saying "in this case?" Because, as it often happens - or it's just me who is looking for cases like these ;) - there are two ways to say "to become." You saw it coming, probably. :D

Now, be very careful - this verb loves a certain preposition - it must be followed by a preposition ל-. Which is not strange at all! If we use a synonym of "become" - "to turn into" we need a preposition as well! You would never say, "she turned stone," - it should be "turn into stone," right? Same here, in Hebrew. :) לַהֲפֹךְ לָאֶבֶן.

And, moreover, this way of saying it helps us better understand what exactly is implied when we say -לַהֲפֹוךְ ל The thing is that it's used exclusively for transformations!

Like in the following examples:
הַצְּפַרְדֵּעַ הָפַךְ לַנְּסִיכָה יְפֵיפִיָּה - the frog turned into a beautiful princess.
יוֹם שֶׁהִתְחִיל כָּל כָּךְ יָפֶה הָפַךְ לַסִּיּוּט - the day that started so nicely turned into a nightmare.
בַּשָּׁנִים הָאַחֲרוֹנוֹת הוּא הָפַךְ לְבֵן אָדָם מְאֻכְזָב וּמְמֻרְמָר - in the recent years he turned into a disappointed and bitter man.

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Attention! לַהֲפֹךְ has also another meaning - to turn over
But when it's used in this meaning no special preposition is needed.

Like in:
הָפַכְתִּי אֶת הַכּוֹס - I turned the cup over.
לָמָּה הַתְּמוּנָה עַל הַקִּיר הֲפוּכָה? - why is the picture on the wall turned (up side down)?

Or the famous קָפֶה הָפוּךְ!
Do you know what it is and why it's called like that? ;) Looking forward to your guesses! ;)