You’re right. If you want to add the individual reading sections, you can add GWW as a new personal resource and give it the divisions you want. Whatever divisions you assign to it will show in the Daily Plan, one at a time.
If you want to leave the divisions as they are (Parts), you can simply tell your child how far you want him to read each time. For example, you might add a Note to the assignment saying something like, “Read three pages today.” Then every time that resource is scheduled and shows on the Daily Plan, that note will print along with it. You can then simply check Worked On each day he reads, and Finished when he finishes a Part.