Hi Barbara, Both of the answers already given are correct, but may not include all of the information you're looking for.
Information about all of the DNA tests used for the confirmation need to be entered on the profiles of the test-takers, and each of the test-takers’ profiles needs to have a privacy level of at least ‘private with public biography and family tree’ so that people not on the trusted list can see the test information.
You then put a statement on the profile for each child whose relationship to his/her parent(s) is confirmed with DNA, explaining how you used DNA to confirm the relationship. Then on the child's 'edit' page, mark the relationship to the parent(s) 'confirmed with DNA'
Detailed instructions for this procedure can be found on these two pages:
https://www.wikitree.com/wiki/DNA_Confirmation
https://www.wikitree.com/wiki/DNA_Confirmation_Example
To answer your gedcom question: You can upload a new gedcom and keep the information already here intact, but you should be sure that your gedcom doesn't include any people who are already here on WikiTree, creating duplicates. If you delete the duplicates first, then you have to go back manually after you've added the gedcom to re-connect everyone. It's often better to manually enter each of the additional profiles.