You can modify the toolbar of CKEditor directly from the template:

https://github.com/inverse-inc/sogo/blob/master/UI/Templates/PreferencesUI/UIxAccountEditor.wox#L126

Aaah yes. Thank you. So for those with the same need :
after copying UIxAccountEditor.wox in /var/lib/sogo/GNUstep/Library/SOGo/Templates/PreferencesUI/, I've added some options from
https://ckeditor.com/forums/CKEditor/Complete-list-of-toolbar-items

This gives to me :
'toolbar': [['Cut', 'Copy', 'Paste', 'PasteText', 'PasteFromWord', '-',
'Bold', 'Italic', 'Font','FontSize', '-',
'JustifyLeft', 'JustifyCenter', 'JustifyRight', 'JustifyBlock', '-',
'TextColor', 'BGColor', '-',
'Smiley', 'Image', 'Table', 'Link']] }"

But, I noticed that the initial "BGColor" button does not appear in the toolbar, nor the "Image" I've added.
Francis, have you an idea why ?

[EDIT]
we can delete "Cut', 'Copy', 'Paste', 'PasteText', 'PasteFromWord', '-'," as the browser doesn't allow pasting this way :(

You should try the toolbar configurator: https://docs.ckeditor.com/#!/guide/dev_toolbar

It will look like this :

ck-options="{ 'autoGrow_minHeight': 70,
'toolbarGroups': [
{ 'name': 'basicstyles', 'groups': [ 'basicstyles' ] },
{ 'name': 'colors', 'groups': [ 'colors' ] },
{ 'name': 'paragraph', 'groups': [ 'list', 'indent', 'blocks', 'align', 'paragraph' ] },
{ 'name': 'links', 'groups': [ 'links' ] },
{ 'name': 'insert', 'groups': [ 'insert' ] },
{ 'name': 'editing', 'groups': [ 'spellchecker', 'editing' ] },
{ 'name': 'styles', 'groups': [ 'styles' ] },
{ 'name': 'mode', 'groups': [ 'mode' ] }
], 'removeButtons': 'Strike,Subscript,Superscript,Anchor,Format,Image,Underline,Scayt,Source,Blockquote,CreateDiv,Outdent,Indent,BulletedList,NumberedList'
}"

[EDIT]
I had some display problems using your example, but I've found that this is commenting old lines ("<!--", "-->") in the block textarea that brokes the UI !
So after replacing old lines from L125 by pasting your example, it's OK : we have a complete editor !

Thank you Francis.

Sorry to reopen this, my question is not exactily about the functionalities but a bug in one in particular : I found that when you want to modify the font color or font background color, if you ask "more colors" there is no "color circle" to customize it.
Maybe (certainly ?) a bug of ck-editor ? Or not ?

Thanks

It works for me. Any JavaScript error?

[EDIT] same thing in the editor when creating a new mail.

[EDIT AGAIN] : sorry, I didn't think to open the Chrome developer tools. Yes, it seems that there are some errors :

colordialog.css Failed to load resource: the server responded with a status of 404 (Not Found)
dialog.css Failed to load resource: the server responded with a status of 404 (Not Found)
colordialog.css Failed to load resource: the server responded with a status of 404 (Not Found)

See https://github.com/inverse-inc/sogo/commit/a31306859551672e86c34ed85f1ff6b8110f9218