Download the file and add to
$ sudo cp Downloads/xmacs /usr/share/X11/xkb/symbols/
Add following configure
<layout>
<configItem>
<name>xmacs</name>
<description>Xmacs</description>
<languageList>
<iso639Id>eng</iso639Id>
</languageList>
</configItem>
<variantList/>
</layout>
to
/usr/share/X11/xkb/rules/evdev.xml
under <layoutList>
.
Navigate to Settings -> KeyBoard -> Input Sources
to find the Xmacs!