This can be done by using the Slur.ms object, using the "dash" option to indicate that the slur does not occur for all verses. In the lyrics, put a "_" character for the second syllable in the verse that has only one syllable. Note that custom objects require NWC version 2.75.
Here is an example:
!NoteWorthyComposer(2.75)
|AddStaff|Name:"Staff"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:12|BoundaryBottom:20|Lines:5|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:1
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Lyrics|Placement:Bottom|Align:Standard Rules|Offset:0
|Lyric1|Text:"Just three _ words\n"
|Lyric2|Text:"There are four words\n"
|Clef|Type:Treble
|TimeSig|Signature:4/4
|Note|Dur:4th|Pos:-6
|User|Slur.ms|Pos:1|Pen:dash
|Note|Dur:4th|Pos:-4
|Note|Dur:4th|Pos:-2
|Note|Dur:4th|Pos:1
!NoteWorthyComposer-End
Mike