There is no option giving the possibility to double tap or hold a key for custom binding.
Of course, i've tried to hold the key when configuring a custom keybind or double tap quickly. But no go.

Also, i think that enforcing shortcut in this window (like the "hold end" to reset all keybind to defaut) is a bad idea for PC. We could need the key ourselve to bind to something, giving that mods allow us to multiply the number of shortcut for specific actions...) I'll rather stay with the "point & click" with mouse for PC rather than sacrificing any key to valid/cancel/default in that specific window (okay we can keep "escape key" to exit the window !)
It may sound "dumb" at first, but with specific keyboard/kb software allowing to easily create macro or additionnal key, i can assure you a lot keyboard keys will be affected to something later on for people playing with countless mod (at least for ArmA 4 which gonna end like Arma 3 on the mod side)