Add button to existing toolbar

Hi all, how can I add a button to an existing toolbat without creating a new toolbar?
For example in myconfig.js I define
FCKConfig.ToolbarSets["mytoolbar"] = [
['Source','DocProps','-','Save']];
with some buttons.
Later in the HTML-document I call mytoolbar:
var oFCKeditor = new FCKeditor('text');
oFCKeditor.BasePath = "/fckeditor/";
oFCKeditor.Config['CustomConfigurationsPath'] = "/myconfig.js";
oFCKeditor.ToolbarSet = "mytoolbar";
now I want to for exapmle add a plugin or another button.
How can I make this?


My motivation is to have a small set of buttons defined once and additional funcrions according to the current situation the user is.

Thanks for help, or a helpful link tor an article in the docus or the forum that I didin't see.

Kind regards, Stefan