Ok, seems like I found something at least:

In fckconfig.js are the following 2 lines:

FCKConfig.SmileyPath = FCKConfig.BasePath + 'images/smiley/msn/' ;
FCKConfig.SmileyImages = ['regular_smile.gif', 'sad_smile.gif', 'wink_smile.gif', 'teeth_smile.gif', 'confused_smile.gif', 'tounge_smile.gif', 'embaressed_smile.gif', 'omg_smile.gif', 'whatchutalkingabout_smile.gif', 'angry_smile.gif', 'angel_smile.gif', 'shades_smile.gif', 'devil_smile.gif', 'cry_smile.gif', 'lightbulb.gif', 'thumbs_down.gif', 'thumbs_up.gif', 'heart.gif', 'broken_heart.gif', 'kiss.gif', 'envelope.gif'] ;

 Changing those doesn't yet give the desired result (namely an extra smiley), but it seems like the way to go. Ideas still very much appreciated.

Thank you for 'staying with' this thread and sharing w/ us your findings.

The important last step: edit the fckconfig.js file that is INSIDE the Custom folder - not the fckconfig.js file inside the FCKeditor folder - that's the step that made the difference for me and really helps a DotNetNuker do this right. 

Since this is a thread about "Smiley Emoticon" and friends, let's bring some of them up on stage: