You can add your own modifiers - from the DCS User manual, pg 25:
Modifiers
Modifiers usually use a key or button combination with Shift, Ctrl, or Alt (by default) or
any custom keys assigned to act as a modifier. For example, you can assign any joystick
button as a modifier and use it to expand the available joystick’s commands that can be
assigned to your input device (i.e. Ctrl + T).
By default, several Modifiers are included in the Modifiers window: LALT, LCTRL,
LSHIFT, MOUSE_BTN_3, RALT, RCTRL, RSHIFT, and RWIN. However, by pressing the
ADD button below the Modifiers list window, you can also create your own.
From the ADD MODIFIER panel, first select the device you wish to set the modifier from.
This could range from a keyboard, to a mouse, to a joystick or a throttle. Once the
device is defined, select the specific key or button on the device from the Select Modifier
Button field. Once complete, press the OK button to save your new Modifier and it will
be displayed in the Modifier list window.
To remove a modifier, select it from the window and then press the REMOVE button.
With at least one modifier in the list, you can now assign a modifier to any Action from
the Add Assignment Panel.