Make a 3-gear bike go easy on uphill
Replacing a hub is not a "simple" procedure -- it, at a minimum, involves relacing the rear wheel, probably with different sized spokes (whose size you must accurately determine).  Relacing is not beyond the abilities of a competent backyard mechanic, but is a skill that needs to be learned.

To add a second chainring you must install some sort of device to take up the chain slack.  A front derailer does not do this, so either a rear derailer or some specialized device.

It would be far easier/cheaper to purchase a new (or "new to you" used) bike.

What you MIGHT be able to do is to replace your front chainring with a smaller one.  Or it's vaguely possible that there is available a replacement larger sprocket for your rear hub.  Of course, either of these would slow down your high gear.