Normal Playlist to Smart Playlist

Smart Playlists are based on criteria, so if there is something common in the Info for all the items in a playlist, you can use that to define the smart playlist.

One option-though I'm not sure what practical use it would have over simply using the regular playlists as they are- would be to select everything in the playlist, GetInfo and in one the fields (Comments maybe) enter some unique value. Then define a SmartPlaylist to match all items with that unique value.

You can make a Smart Playlist that will match songs in other playlists, but aside from combining tracks from different playlists into one, there's not much value in this either.

Is it possible to easily convert these normal playlists to smart playlists
Why would you want to convert them? It doesn't make sense to convert them to a smart playlist, especially since they are already in a playlist.

You don't add songs to a smart playlist. You add criteria and iTunes selects songs that maatch your criteria.
A Smart playlist will pick songs automatically based on criteria such as Last date played, Rating or Date added.