Okay, I decided to resolve this myself. This is the long way around the barn, but it gets me to the end result.
On your mobile device that has the notes created with a client other than iCloud, open Notes. Click on Accounts and select any account other than iCloud. You will now see all the notes created via that account. Go to each note and email the note to yourself using your iCloud mail address as the destination. Repeat this process if you have additional accounts other than iCloud with notes. Repeat this process for other devices if you have notes on those devices not covered previously in this step. You should now have all your non-iCloud notes in your In Box in iCloud Mail.
Open iCloud on the iMac. From the initial screen showing the iCloud apps, select Notes and open it in another tab. Then, open Mail.
Select one of the notes in your In Box in Mail. Highlight the content of the note and Copy. Go to the tab where you have iCloud Notes. Click on the "+" sign to open a new note. In the EDIT menu in the Safari menu bar, select Paste and Match Style. You will now have created a new note in iCloud Notes to match the one you sent to yourself.
Repeat this process for all the notes you sent to yourself via iCloud Mail.
Go back to your mobile devices from which you retrieved the non-iCloud notes. Again, select Accounts and go to the non-iCloud accounts. Delete every note in these accounts as you now have duplicates on your device.
You now have all your notes on all devices courtesy of iCloud. Of course, you have also designated iCloud as the default client for future notes to prevent having to do this every again. The only downside, and it's not much, is that your notes' "origin date" will appear to be the time today when you made the transfer happen instead of when you originally took the note. However, if and when you change or append any note, the date stamp changes to the time of the revision, so this isn't a big deal.
I had hoped to avoid all this with some sort of transfer solution. None being forthcoming, I hope this helps anyone else wanting to transfer notes to iCloud notes.