Just on this example, you can physically pick soil up and move it. Enough for 9bn people? No. Enough for 100k? Yes.
If the sea is fertile the soil can be hacked to be fertile. Any seabird could tell us how
But this is not the problem. The sea level will raise
Edit: good point in the next comment, expect sea levels rising for at least 10m.
I'm not saying it's a good idea, I'm saying it's a theoretically possible idea in the face of an extinction-level event.
(I'm trying to find a sea rise map that looks right by eye and not having much luck. Here's the North Atlantic at 10m[1]. Surely that's not enough flooding?)
[1] https://coastal.climatecentral.org/map/4/-10.3532/51.4503/?t...