You can use TVInfo(SeasonDisplay), it'll automatically prefix "Season" to numeric values, and leave everything else alone.
So if you have "1" in the field, it'll be "Season 1", but if the Season field contains "Specials", it'll end up as "Specials".
It won't convert empty or 0 to Specials though.
In some shows the special episodes belong into the normal episode watching order, so instead of having to remember when to watch which special, I typically just include them in the normal season, in the proper spot where they need to be watched - I change the Season/Episode after looking up the info from TVDB.
This is typically for british shows, like Doctor Who or Downton Abbey, where the christmas specials are just "normal" episodes that should be watched between the seasons.