I was working on a tooltable inside mach4 (i wanted to avoid popping dialogs) and wanted to add the cutlength of a flute to the tool table.
So from main menu of mach4 enter "view->tool table", which opens the mach4 tool table dialog.
From there go to "Edit -> Table Fields", which opens another dialog with a notebook control.
Last tab to the right is called "User Fields".
There you can add fields to the tool table.
To retrieve the values from lua there's a mach4-API that takes a fieldname as parameter:
mc.mcToolGetDataExInt(inst, toolNumber, userdefinedFieldname)
That API has variants for Double and String too, so straight forward

See attached picture of my tool table tab page. Column "SL" is a user defined field from mach4 tool table and I don't have to care about storage

hth Reinhard