What you are describing does not seem to be standard engraving practice for extender lines. Can you post an example clip or screen shot that shows what you are trying to do?
Perhaps Melismatic can be coaxed to do what you are asking, by using a hidden muted grace note:
!NoteWorthyComposer(2.7498)
|AddStaff|Name:"Staff"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:12|BoundaryBottom:16|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:"Oo\nAh\n"
|User|Melismatic.nw|Pos:0|Class:StaffSig
|Clef|Type:Treble
|TimeSig|Signature:4/4
|Note|Dur:Whole|Pos:-1^
|Bar
|Note|Dur:Whole|Pos:-1
|Note|Dur:8th,Grace,Slur|Pos:-1|Opts:Muted|Visibility:Never
|Bar
|Note|Dur:Whole|Pos:-1^
|Bar
|Note|Dur:Whole|Pos:-1
!NoteWorthyComposer-End