How do I add metadata to files I've already converted without converting them again?

There are three ways to do this....

For an individual file you can drag the file into iVI, double click on it in the iVI main window, set up the metadata the way you like it and then press the Save Changes in Current File button.

In order to modify many files at once you first load the files into iVI. Then get the metadata set up for them - you can change multiple values at once by selecting all of the files and right clicking and selecting "Edit" or do a more detailed edit on each file individually. Then drop the HD/SD box down on the main window for each file and select either Info Only (again you can do this for them all at once by selecting them all and right clicking and selecting Set Conversion Type>Info Only) or INFO & MOVE. INFO ONLY will add the metadata to the existing files but otherwise leave them the same. INFO & MOVE will add the metadata and then rename and move the files in accordance with your settings on the Converted File tab of iVI's preferences.

Then when everything is ready press the Convert All button and the metadata will be applied to the list of files.

Keep in mind that the files will need to be of m4v or mp4 file type for metadata to be applied to them in this way. Other file types will need to be converted first.