This.. sort of works, but it isn't the best idea for what you want to be doing for print.
You can't "convert" RGB into CMYK, since RGB isn't really a colour profile, you can turn it into some sort of colour slurry like people do when they open Photoshop, but you're not really achieving CMYK colours, your prints are going to look different if you do it this way.
If you're using GIMP anyways, just get this:
http://www.download32.com/cmyk-plugin-for-the-gimp-i76600.html
Scribus can import tiffs, which will cut out step 2 of your method, plus it will get you closer to the CMYK separations you want. Just keep in mind, unless you start something new in CMYK, you're not getting perfect CMYK. Like I said, it's more like a slurry of colour and you'll get something a little bit different no matter what unless you start with a perfect CMYK image and work from the ground up, or use pantone colours, which will be tough with freeware.