I always used Photoshop to place a logo or still in a RGB colorspaced file for use in FCP but I wondered how others (specifically those that don't have Photoshop) go about this?

Is there another quick way to convert CMYK files to RGB?

Try using Preview, in the Tools section you can choose different color spaces although it seems to be finicky with CMYK. Another hint I have seen but never tried is to import the CMYK image into iPhoto and export a JPEG. It is turned into RGB in the process.