NConvert currently does not have an equivalent of the XnView MP browser function Create > Join images..., which might usefully be added when time allows, if it ever does...
Recent request: How to stitch 2 images vertically or horizontally using nconvert?
A possible temporary workaround when the same operation is to be performed repetitively, is described in these posts:
Re: Joining Images in XNViewMP
Create a contact sheet
'Join images' currently not supported
Moderators: XnTriq, helmut, xnview