You may import custom fixture symbols into your show file for use with the Magic Sheets feature. The image must be in the SVG (Scalable Vector Graphics) format, and you will have to tag the "base" and "outline" layers in order for the selection and fill/outline coloring to work properly.
Below is a Step-by-Step tutorial for creating a custom fixture symbol from scratch using the popular, free SVG editor
Example 1 (very simple, result of Step-by-Step tutorial)
Example 2 (larger selection indicator)
Example 3 (alternate dot selection indicator)
Example 4 (Fixture outline, where you can link the fill color to the channel color)
Example 5 (entire fixture shows selection state, can you can link outline color to channel color)