How do I drill a long hole lengthwise all the way through a post to make a standing lamp?
Long (18" and longer) drill bits do exist. If the hole is wide enough, bit extensions are also usable.

However, there's also a simple-but-elegant cheat. Cut the piece lengthwise, rout a channel in one or both sides, glue back together. If you make a thin-kerf cut, and are careful during reassembly, the glue line can be nearly invisible -- especially on straight-grained wood with the cut aligned with the grain lines.