You can add custom symbols to the "Insert Symbol" dialog by defining the corresponding entities in your "config.xml". For example, in order to add the symbols "%u0393" and "%u0394" to your "Insert Symbol" dialog, you'd have to add the following code to your config.xml:

<presetsymbols>

<symbol entity="&#913;"/>

<symbol entity="&#914;"/>

</presetsymbols>

You can add additional symbols to the dialog by adding additional entities through the use of "symbol" elements.