In the recent past there was a command in Surface - DTM tools that was 'Insert Breaklines', I use this regularly when editing DTMs from a survey rather than re-create the DTM, especially if I have been swaping triangles etc. This command no longer exists.
In the DTM Editor there is a tab "Surface Data". On this tab there is an Add Data option. Use this option to select linework that you want to add as breaklines. Points are also supported. For example you have an existing dtm and you want to add points outside of the boundary of the dtm to the expand the dtm. See attached video for example.