Good morning! You needn’t add your import to all of your existing audience groups, but it does need to be added somewhere. I suggest creating a new audience group and importing into that group, selecting the import method Add & Update. This will import your members into that audience group and update any existing members’ information across the board so that all the data in your spreadsheet updates for all the matching members in your audience.
Then, you can delete that new group you created, which will only delete that group of members and not the members themselves.