The reason that Pantone colors even exist is that most of them cannot be reproduced with CMYK. Converting the process colors thius is not a good idea.
Unless you are in packaging or fashion, where for workflow reasons the Pantone extension might be necessary, the only thing you really need for working with spot colors are the printed Pantone swatches books. Spor colors are about printing them. And if you need the equivalent of them on screen, you still need to know how they look when printed.
Unfortunately they are really expensive and the extension doesn't accompany them.