They already do import the water from external sources, mainly the Cutzamala system, which is not quite enough as there are parts of the city that has historically suffered from lack of water supply, like Iztapalapa.

The city is indeed a valley, with an altitud of more than 2k meters above sea level, so meteorogical inversion is a given condicion,

A fix to their issues is to bluntly stop growing (it is expanding), and decentralize the gubernamental institutions there so their related weight can be sparsed along the country, but that is a political debate that may never be opened to discussion

Edit: forgot to mention the heavy rains periods. Being inside a valley, on top of a former lake, and an almost impermeable surface, you can now begin to imagine why the city floods on the rainy season. The sewer system is decades old and in a really bad shape, so the rain mostly has to wait for evaporation (good luck for that during an inversion event)

I am glad not to live there anymore.