Translation:That additional tomato is improving the dinner.
Dinner is the main meal, whether at midday or in the evening. That is what makes the problem - many persons have their main meal midday - they have breakfast, dinner, and supper. Many persons have their main meal in the evening; they have breakfast, lunch, and dinner. Dinner is the main meal - either lunch is the smaller meal at noon or dinner is the smaller meal at night.
There are accepted translations that include "makes ... better".
What is the exact sentence that you wrote? Did you get the word order in your English sentence incorrect, perhaps?
Please - always ask about entire sentences, not about individual words! Often the problem is with word order or with some other word (e.g. a gendered article rather than the noun), and if you only post the word(s) that you think are the problem, you may not get a useful answer.
Would you actually use "zusätzlich" in conversational German
Yes; in a situation such as die zusätzliche Tomate, it sounds fine to me.
If you're asking for one more, though, I'd say noch eine Tomate -- but you can't put die in such a situation any more than you could say "the another tomato".