I have seen a device called a coolant inducer that converts regular machines to "through spindle coolant". Of course, the coolant doesn't actually go through the spindle, it's piped to the inducer. I recall seeing one that would fit in a toolchanger. This used a block of sorts mounted to the machine spindle casting that carried the coolant. When the inducer was put in the spindle, a pin fit into this block making a pathway for the coolant. Think of the restraining pin for a tapping head - the same idea.
I don't know if these are even available any longer.