I kept digging and discovered I have to buy the AutoChart Pro software if I want to do this. This was on PP 19 of the AutoChart Pro manual:
Create tracks for i-Pilot Link
You can easily create a track for use with i-Pilot Link.
• To create an i-Pilot Link track from a contour: Hold down the right Ctrl key and double click the contour you want. The contour will now be highlighted. To export it to a track file, use Tools/Convert Selected Contour, and save in ‘i-Pilot Link HIT track (*.hit)’ format.
• To make an arbitrary shaped track, first create a line with Tools/Draw Line - select Marker line/Polyline as data type when saving the file. Your line will be saved in AutoChart .acd format. Then convert your .acd file to a .hit file using Tools/Convert Line to Track.
To load your line to your Humminbird unit:
1. Place the .hit file on a memory card in a folder named ‘MATRIX’.
2. Put the card in the unit after it has been turned on, you will be prompted to load the data.
I ordered the software
(it's only money) but, if previous experience is an indication, I suspect the software won't import the paths I've already created and I'll have to draw them again. We'll see...