https://www.ufopaedia.org/index.php?title=ICONS.PCKmy approach would be along these lines:
shrink the reserve TU buttons horizontally by approximately 33% each
this would make space for a third set of buttons.
one could be the "reserve for kneel" button
the other could contain maybe a "no reaction fire" button or a slider or something.
then i'd fiddle with source to alter the bounds of the buttons and so on.
of course, this would be my approach as a MODDER.
i'm guessing you're looking at adding this as an "optional feature" in vanilla OpenXcom, plus you care about copyright.
my suggestion in this case would be:
make a new .pck file (that's not a copyrighted format, right?) that contains ONLY the shrunk down/modified buttons, superimpose it over the vanilla buttons and have an alternate set of boundaries, if "extra time unit reservation options" is enabled in the menu.
i'll throw something together and post a link here.