I have done it in the past manually, it's not trivial. Best way is to edit the g-code and reload, trim away everything after the preamble to the last plunge move.
It's not possible to just resume the file from the point it ended as that is most probably right in the workpiece without a safe direct line from the tool height probe position. So we need at least a safe ramp/plunge movement. And the preamble needs to run again, to use the correct tool height offsets, tool diameter compensation, bring the spindle up to speed...
I don't think there can be a generic resume-at-position for this that fits all. Some tools that have a certain orientation or don't enter the workspiece colinear to the spindle, e.g. t-slot cutters, profile cutters in general, taps. And then there's five axis machines.
