In n4ce 4.30 and above we have introduced a new feature which allows users to add points into the DTM which are not currently within the DTM boundary.
Extending a DTM
To extend the DTM first add the point to the list. This can be done via selecting List --> Add tools. In this case, I am going to use List -->Feature --> Line.
Once the Points have been indicated we can now select DTM --> Extend.