Here's a symbol we created by grouping three shapes together:
Here's one we made by using the polygon line tool.
Next, you'll want to create a custom library and your new symbols to it.
In the SmartPanel, click on the Symbols tab and under More drop down choose "Add New Custom Library."
In the dialog that appears, name your library.
You'll now have an empty library docked next to your drawing area.
To add your symbol, drag it from the drawing area over the library until you see the cursor change to the add symbol shape.
Now, you can name your symbol and choose whether you want it to change colors according to the diagram's theme or keep the colors you've designed it with.