FilterForge, CMYK colors

One of the POD book companies requires that all spot colors used on book covers be in CMYK, and that for the density, the CMYK total value should not exceed 240 percent.

I have worked with a different POD company before and did a few book covers that had illustrations that had been tweaked in FF (the covers were laid out in Serif PagePlus). I did not convert the illustrations to CMYK, and I did use really dark colors on one of the books that must have surpassed the 240 percent CMYK values. They printed OK, although from printing to printing the color varied some.

If I specify that the overall PDF be converted to CMYK when I make the PDF in Serif PagePlus, is that good enough? Or do I need to convert the photographs or illustrations to CMYK after I have run them through Filter Forge?

If I do need to convert the pictures, does anyone have suggestions for how to do that? I looked at a free program called Delaboratory that seems to convert pictures from RGB to CMYK -- is that a good option?

I don't have Photoshop. I do have PhotoFiltre Studio (which doesn't seem to convert to CMYK) and RawTherapee, but I am not experienced with RT enough to know the full capabilities. So far, it looks like it has three color possibilities (LAB, RGB and onte other) but no CMYK.

I have been thinking about getting PhotoLine because I think it is compatible with FilterForge, but I am fairly used to doing layouts with Serif PagePlus (unfortunately, the company has stopped working on that program, so eventually I will have to do something). I know about Scribus and Gimp, but I think the learning curve would be too great to easily switch.

I have a wonderful book of CMYK samples called The Complete Process Color Finder, so I may be able to eyeball whether the colors I am using are in an acceptable range. (Delaboratory will generate a palette of the colors used in an image, but I haven't been able to figure out a way to export the palette and check the colors for their values.)