Software can be maddening when it does what you tell it to do rather than what you want it to do.
I wouldn't. The performer needs to see this Key change. Something like this:
!NoteWorthyComposerClip(2.0,Single)
|Clef|Type:Treble
|Key|Signature:Bb,Eb,Ab
|TimeSig|Signature:Common
|Chord|Dur:Whole|Pos:-2,0,2
|Flow|Style:Segno|Pos:10|Placement:AtNextNote
|Bar
|Chord|Dur:Whole|Pos:-4,-2,1
|Flow|Style:ToCoda|Pos:10|Justify:Right
|Bar|Style:Double
|Key|Signature:C|Visibility:Never
|Key|Signature:F#
|Chord|Dur:Whole|Pos:-2,0,2
|Bar|Style:Double
|Key|Signature:C|Visibility:Never
|Key|Signature:Bb,Eb,Ab|Color:1
|Flow|Style:DSalCoda|Pos:10|Justify:Right|Placement:AtNextNote
|Bar|Style:Double|SysBreak:Y|Visibility:Never
|Flow|Style:Coda|Pos:10|Justify:Right|Placement:AsStaffSignature
|Key|Signature:Bb,Eb,Ab
|Chord|Dur:Whole|Pos:-2,0,2
!NoteWorthyComposerClip-End
(color added for emphasis)
If there is a change of Clef or time signature, the perfomer will need to see that as well.
NoteWorthy doesn't do a very good job of setting Codas off from the main body of music. Sometimes post-processing needs to be done as in the attachment.
edit 2009Apr20: Changed KeySig order and some placement options in clip.