If your H1 is expired, AND you did not file for an extension, AND you already applied for I-485, the only way you can travel is by using Advanced Parole.
At POE, you will get a new I-94 stamped with "Paroled until ....".
And you're right .. I-539 is for non-immigrants.
Pending adjustment is kind of 'in limbo'. You are not a legal resident/immigrant yet, but you don't fall in any of the non-immigrant visas either. Sometimes, the general advice for people with pending adjustments is: "Do not leave the country unless it is absolutely necessary"

Get an AP if you really have to/want to make a trip outside the U.S.
So .. back to your question if you can or need to extend I-94? No - you can't if the H1 is expired.
On the other hand, even if your H1 is still valid, and you submit I-539, the validity of your I-94 will be only UP TO the validity of your H1. What's the point of wasting money here?
Once your H1 is expired, the only proof that you are in-status is your ORIGINAL I-485 receipt. Keep it in a safe place!