Insert Triangle 
Insert Triangles
Use the Insert Triangle option to add extra triangles (facets) to an existing triangulation. New triangles are defined by indicating either existing design points and/or triangulation vertex points.
Instructions
On the Model menu, point to Triangle Surface, and then click Insert Triangle.
Select the triangulation. The triangulation model will be selected automatically if there is only one loaded onscreen.
Digitise the vertices of the new triangle. Use the Snap to Objects,
Snap to Points or
Keyboard Input modes for accurate selection.
The triangle is then created.
Digitise the vertices of another triangle. Cancel when you have finished adding triangles.