Lengthen the throat on your drill press

I have not seen longer columns for sale.

Like you say , it is just a pipe.  You might find a length of pipe ( structural tubing actually ).  Pipe measured ID Tubing measured OD.

You may have trouble finding the right size and trouble getting it threaded or  ^ $.

Some things to consider:

As the column gets longer , to have the same rigidity to resist flexing, it needs to get larger in diameter or a lot thicker wall.

If you use it under light loads then no big deal.  If you use large diameter and bits that are less than perfectly sharp then I would not recommend the same size tubing but longer.

Drill presses are dirt cheep compared to other tools.  A big honking floor model is about the price of a medium size hand plane.  Go figure.  How does that work ?

It is great to have more than one drill press and of different sizes.

Then if you buy used it is even cheeper.

It is true you can drill off the side of a bench.  That works great.  Be sure every thing is stout.  Beyond stout if you want any kind of accuracy.  Think 4x4s, thick steel plate and what not.  If you through bolt to a 3/4 plywood bench top that isn't really going to be so great.

Here is one more thing to consider . . .  if you use a band saw to resaw a floor model drill press makes a stellar adjustable out feed table.  Turn the table to the side and bada boom bada bing.

PS: gosh they have gone up some.  I have basically this one

http://www.toolking.com/delta-17-950l-16...

With out the silly lazer thing and mine has a round table.  ~ $ 380  a couple three years ago and I think that included shipping.  You may find something similar if you shop or buy used and local.