If you cut the pex line, you should be able to throw on one of these: https://www.supplyhouse.com/Bluefin-PAMA050-1-2-PEX-AL-PEX-Compression-x-1-2-Male-Threaded-Adapter

And then add this valve on it: https://www.supplyhouse.com/Bluefin-SSQ050F-038CS-1-2-FIP-x-3-8-OD-Comp-1-4-Turn-Straight-Stop-Valve-Lead-Free-Chrome

And you're done. Probably assemble the valve and the pex compression with tape/dope before putting it on the supply line.