The time line would be too long for the time line of the “7 year warning” of Joseph’s dream, unless there were expedients we know nothing about today.
Hollowing out an existing pyramid might have been easier, a repurposing, and maybe Joseph might have suggested this if it made sense... but again, volume calculations.
I think hollowing out a pyramid would weaken and collapse it. They actually weren’t that great in terms of structural engineering... just well-placed piles of stone. I’m not denigrating the skills of the day, they were good for what they were. But they weren’t up to the task of building a large hollow structure built of stone. All those extremely heavy stones had to be resting on other extremely heavy stones. Too much hollowing and the walls definitely come tumbling down.