Could Mogami have been built to accept more armour?

Could the Mogami class have been designed for a later armour upgrade as well as its planned 8-inch-gun conversion?

Yes. Added armour would have reduced speed somewhat, but the ships had enough power to remain very fast.

The later armour additions made to British County-class cruisers offer a model for designing growth margin into Mogami. The Japanese ships had ample machinery power, so additional protection need not have made them slow cruisers. Later bulges reduced speed to roughly 35 knots, still a high figure for a better-protected ship.