Doesn't seem to be, no.
With non-meal-plan items you've entered, when you click the item you can change the value, but in meal plan items the click means "swap" so you can't access that feature.
That's a bit annoying. Oh well, I guess consider it like the odd times you might not have the meal plan suggestion at all but have something else entirely. Like if it says half a piece of fruit you might have some string cheese instead. So you need to add that manually. I guess we just need to consider "a whole piece of fruit" as "a different thing entirely" to "half a piece of fruit".
Deb, in New Zealand