Hi Jose,
Thanks for your interest in Syncfusion products.
Regarding customizing SymbolPalette, please refer online help documentation
http://help.syncfusion.com/ug_83/User%20Interface/Silverlight/Diagram/ and navigate to Concepts and Features > Symbol Palette > Customizing the Symbol Palette. We have also represented CutomSymbolPalette in ‘Features Demo’ sample from Dashboard, where Palette can be switched between Custom and Default SymbolPalette using the MenuItem labeled ‘Custom Palette’.
Please let us know if there are any concerns.
Regards,
Jegan