Generally, you can directly prescribe what configuration of each of basic xkb components should be used to form the resulting keyboard mapping. This method is rather "brute force". You precisely need to know the structure and the meaning of all of used configuration components.
This method also exposes all xkb configuration details directly into xorg.conf configuration file which is a not very fortunate fact. In rare occasions it may be needed, though. So how does it work?
There are five basic components used to form a keyboard mapping:
Look at the following example:
Section "InputDevice" Identifier "Keyboard0" Driver "kbd" Option "XkbKeycodes" "xorg" Option "XkbTypes" "default" Option "XkbSymbols" "en_US(pc104)+de+swapcaps" Option "XkbGeometry" "pc(pc104)" Option "XkbCompat" "basic+pc+iso9995" EndSection
This configuration sets the standard X server default interpretation of keyboard
keycodes, sets the default modificator types. The
symbol table is composed of extended US keyboard layout in its
variant for pc keyboards with 104 keys plus all keys
for german layout are redefined respectively. Also the logical
Control keys is swapped.
The standard keyboard geometry (physical look) is set to pc style
keyboard with 104 keys. The compatibility map is set to allow
basic shifting, to allow Alt keys to be interpreted and also
to allow iso9995 group shifting.