If I'm understanding your problem, you created this xml file in some other package, and want to do a spell check, so want to open it in Word?
What happens if you simply paste your xml file into Word, and run the spell check as-is? If it's raw xml with open/close tags, maybe you can choose to ignore those during spell check, or add to dictionary. Not sure if this is helpful as you mentioned you want to create a tool to do this.
If these ideas don't help, can you post a portion of your xml document you're talking about so we can take a look at how this might be done?