They wouldn't be playing on the same field as the Mets per se, they'd wheel in a big *** piece of turf/grass and lay it down over the diamond.
These transformations can happen quick and easy. Yankee Stadium does it every year for the Pinstripe Bowl, and Citi has had international soccer games before. Hell, in the Carrier Dome here at Syracuse, they assemble a basketball court and bleachers on 1/3 of the football field in a few hours at the most. They have it down to a science.
Logistics wouldn't be too hard.
Pic of what the layout looks like: