I ran into an issue when trying to add emoticons to already existing packs. I would unzip the pack, rename the folders to something different like v2, edit the plist file and zip. The problem was when I went to add it in Spark the icon preview window went blank and I had to exit Spark and edit the spark.properties file to get everything reset again.
It appears my issue was on the zipping of the file. If I never unzip the pack and just add my icons and edit the plist within the archive the emoticons appear.
Not sure if anyone else ran into this trying to add emoticons to already existing packages but thought I would post if it saved someone some time. I tried to unzip/zip with winrar and 7zip with the same results. Anyone that knows why this happens I would be curious to find out.