There is no true software-based conversion or matching method for CMYK to spot.
Sample-print their CMYK colors on your equipment an use a fresh swatch book to find matching spot colors.
You can do it with Photoshop. Click the Foreground color picker, enter the CMYK values, choose Color Libraries, select the Pantone book you want to reference, and the closest ink will be displayed. Keep in mind that the appearance of CMYK depends on the document's color profile or the Color Settings' CMYK Working space if nothing is open. So, for example US Sheetfed Coated will likely return a different Pantone color than US SWOP Coated.
And there's a fairly obscure "fixup" in acrobat that allows you to substitute a spot color for CMYK, though as I recall you must identify the spot first.