Okay, so I was able to remember how to set "import and sort" so that non-media files can be imported. This can work. It is possible to import html files (and so probably other files?) using this method. I have only used this with html files.
With XNVIEWMP open:
1) Goto "Tools"->"Settings"->"Browser"/"File list"
The "Settings" dialog will open.
2) Click on "Custom filter" tab.
3) Find the "Video/Other" field. Add the desired file extension for the file that needs to be imported, making sure the extension is separated from the other entries already there with a space.
This will tell the program to treat the file that needs to be imported as a video file.
4) Close out of "Settings" Dialog.
5) Go to "Tools"->"Import and sort ..."
The "Import and Sort" dialog will open
6) Set the Source/Destination/Photos/Videos and other settings as desired.
7) Optional: If you want the html or other file to be renamed using the jpg renaming convention, make sure the "Videos"/"Rename like photos" box is checked.

Optional: If you also want the file in the "Other FIles" folder, make sure the "Destination"/"Transfer files with no exif to 'Other Files' folder" box is checked.
Using this setting, the file will be copied to "Other Files" and will also imported as if it were a video file.
9) Start the import.
10) Depending on the filetype you needed to import, and your other uses of XNVIEWMP, it might be necessary to reverse what you did in 3) - go back to "Settings" and take the ext out of the "Video/Other" field. Since I mainly use the program for importing, leaving the html ext in "Video/Other" works for me.
- If you find you do need to reset this often, an alternative is to create a special xnview.ini file (where all the settings are stored) with the "Video/Other" setting as you need it, and make a batch file which copies the special .ini file into xnview before every special import, starts the import and after the import copies a regular xnview.ini file back in place. I use several batch files for doing different file renames/different subfolder names depending on type of pics to be imported.
If anyone needs a sample batch file to look at which does this, I can share that.
Happy importing,
Craig