Can the ultrafilter in the water purifier be replaced with a reverse osmosis core?

You can't. The water pressure is not enough to penetrate.

In the process of ultrafiltration, under the pressure, the aqueous solution flows through the membrane surface, and the solvent (water) smaller than the membrane pores and small molecular solutes permeates the membrane becomes a purified solution (filtered solution), while the solute and solute groups larger than the membrane pores are intercepted and discharged with water into a concentrated solution.

Reverse osmosis core:

A membrane separation operation that uses pressure difference as driving force to separate solvent from solution. When pressure is applied to the feed liquid on one side of the membrane, when the pressure exceeds its osmotic pressure, the solvent will reverse the direction of natural permeation.