Update: Looks like I was late to the game on this as well: http://www.intec.co.uk/xpages-8-5-2-rich-text-extending-the-ckeditor/

A while back, I got annoyed by the lack of a "Source" button in the default rich text editor in XPages. After a bit of digging, I found that there is indeed one (not to mention a whole world of CKEditor toolbar plugins and enhancements) - just not by default. I think there's a way you can pick from a couple named "stock" toolbar layouts, but I ended up going the route of defining my own, naming each button in a group.

In CKEditor, at least pre-3.6 (and I guess it works later too), toolbars can be defined inline via a JSON array-of-arrays in the "toolbar" property of the editor. A hopelessly simple toolbar definition would look something like this:

[
	["Bold", "Italic", "Underline", "Strike", "-", "TextColor", "BGColor" ],
	["Indent", "Outdent"]
]

That would define two button groups with the named controls present. The "-" is a way to manually place a separator - think of them like non-breaking spaces: they look the same as the divider between groups, but will not cause the group to be split onto a second line if the toolbar is too wide for a single row.

For my purposes, I went a bit nuts:

[
	["Format", "Font", "FontSize"],
	["Bold", "Italic", "Underline", "Strike", "-", "TextColor", "BGColor", "-", "JustifyLeft", "JustifyCenter", "JustifyRight", "JustifyBlock", "NumberedList", "-", "BulletedList"],
	["Indent", "Outdent"],
	["Subscript", "Superscript"],
	["RemoveFormat", "-", "MenuPaste", "-", "Undo", "Redo", "Find", "LotusSpellChecker", "-", "Image", "Table", "Link", "Flash", "-", "PageBreak", "HorizontalRule", "SpecialChar", "Blockquote", "Smiley", "ShowBlocks"],
	["Maximize", "Source"]
]

Inside an XPage, you can set this using the "attrs" property in 8.5.3 - though "dojoAttributes" would probably work fine in 8.5.3 and earlier as well:

<xp:inputRichText value="#{doc.Body}">
	<xp:this.attrs>
		<xp:attr name="toolbar"><xp:this.value><![CDATA[
			[
				["Format", "Font", "FontSize"],
				["Bold", "Italic", "Underline", "Strike", "-", "TextColor", "BGColor", "-", "JustifyLeft", "JustifyCenter", "JustifyRight", "JustifyBlock", "NumberedList", "-", "BulletedList"],
				["Indent", "Outdent"],
				["Subscript", "Superscript"],
				["RemoveFormat", "-", "MenuPaste", "-", "Undo", "Redo", "Find", "LotusSpellChecker", "-", "Image", "Table", "Link", "Flash", "-", "PageBreak", "HorizontalRule", "SpecialChar", "Blockquote", "Smiley", "ShowBlocks"],
				["Maximize", "Source"]
			]
		]]></xp:this.value></xp:attr>
	</xp:this.attrs>
</xp:inputRichText>

It looks like the syntax changed for 3.6, but not too much.

If you want to look into more specifics, you can take a look at the examples and un-minified code in (data)/domino/html/ckeditor/_samples and _source.