Hi Bean I am not any form of expert in Photo Paint X6! although I have used older versions a little. I know that your version does have colour management in it and when you export your pictures there will be some of of gamut colours from your original photos. Now this is quite a complex problem to give to you through this forum but I would talk to your printer and get a trial copy printed as it won't cost that much if he uses a digital printer. Most large format printers use RIP software to pull your photos from your PDF file and this software can handle just about all of your colour that you have used in your paintings well. The PDF file that you have compose in Corel PHOTO-PAINT X6 should have all the information needed for this software if you set up the PDF export correctly. Don't worry about the CMYK rich black colour the printer recommended unless you have paced text on your PDF file. Talk to your printer and explain it's a photograph and tell them that you have not corrected "out of Gamut colours" and ask to have one wet copy. If on your desk you have a laser printer then get your wet copy from that but print it from Adobe Reader instead of using Corel PHOTO-PAINT X6.

You can learn about Out of Gamut Colours from Google and once you understand that then use Google again but make your search related to Corel PHOTO-PAINT X6 and out of gamut colour.
Hope that helps