Can I append my new list to my existing list?
Yes, this can be done easily in fact.
How?
When adding to a list from a file, choose "Add to existing" after uploaded and choose the existing list you wish to add to.
What does that mean?
This means that if you have a list with three columns that are: Email, First Name, Last Name, it would be important to have the same fields in the file you are adding to that list to avoid formatting problems.