To creating your own SI5 Visio shapes, please check the following link:
Custom Visio Shapes - D-Tools Documentation Wiki
Save your stencil to the following location:
Win7/Vista: C:\ProgramData\D-Tools\SI5\Visualizations\Stencils
XP: C:\Documents and Settings\All Users\Application Data\D-Tools\SI5\Visualizations\Stencils