I work in a school and have covered lunches fairly often. You daughter could conceivably have both school lunch and food from home, but that would be a lot for a kindergartener to manage in a short time frame. Once she goes through the lunch line, there's usually only about 15-20 mins left for eating, and kindergarten kids are notoriously slow eaters. It's likely that whatever extra you pack will go uneaten or get thrown away.
A better option might be to let her pick one or two of her favorite school lunches per week, and round it out with extra fruit, veggies or whatever at breakfast or for an after school snack on school lunch days. That way you know she's really eating the healthy food!