Would it be possible for a planet to form so it has all of the landmass in only one hemisphere?
By landmass, I mean all landmass that is above sea level.
And human-habitability is encouraged...
If you divide the world into two hemispheres, with one centered in France and the other on the opposite side of the world, around New Zealand, then the "Francocentric" hemisphere has about 7/8 of the world's land. So the Earth is already pretty close to the situation you describe. I'd say that makes it possible. See, for example, https://en.wikipedia.org/wiki/Land_and_water_hemispheres.
In a Sense
Think Pangaea. Back around 300 million years ago, all of our continents were one mashup of separate landmasses.
This might not be what you are looking for, as you do not state whether or not you mean eastern/western, or northern/southern. Pangaea was all in the western hemisphere, or eastern, depending how you look at it.
As for the Northern and Southern Hemisphere
There is no reason that the major landmasses could not form to the north or the south of the planet.
It all depends on the movement of the tectonic plates. Maybe in your story the continents were all dispersed but eventually they moved back together, as they will someday do on Earth.
How planets form is subject to complex events on astronomical and geologic time scales. See the nebular hypothesis for a detailed description, but rocky planets like Earth start out as a warm mess of spinning gases and dust that coalesce and cool into relatively homogeneous balls of spinning rocky matter.
Those planet embryos (actual term) initially have almost no atmosphere and as such have no defense against the numerous large (moon- to planet-sized) and smaller asteroids impacting the surface.
Even if the combination of these initial processes and plate tectonics result in continents (or a Pangaea-like supercontinent) that span multiple hemispheres, continental drift and further large impacts would continue to reshape the continents over millions of years.
The percentage of your planet that is covered in water (or other liquid ocean, I suppose), and the number, mass, and orbit of natural satellites (moons) will also be an important contributor. Although simply adding oceans won't itself affect the position of continents, it will reduce their size and number. It would be rather unlikely to have an entire hemisphere without a continent if your surface is only, say, 30% water, for example.
The Earth is already pretty close. The Pacific takes up almost a whole hemisphere. Here is a screen shot from Google Earth. I just rotated the model manually trying to show the maximum amount of water I could manage. It's not hard to imagine a planet with a whole hemisphere of water.
Mars now has not much water... but if it once had an ocean it would have covered most of the northern hemisphere, while the southern hemisphere was mostly high and dry (with a couple of rather large lakes)
The difference between the hemispheres can be seen on this map from NASA. Roughly, green and blue could have been sea.
I don't see why not? It could largely depend on how much water is on the planet. If you have a planet like the moon Titan, the entire surface is miles and miles of deep water. So you wouldn't want quite that much.
As Vincent pointed out in the comments, Pangaea was theorized to be all the current continents together in one spot, and depending on how you count your 'hemisphere', (North-South, or East West meaning just one half of the sphere) it was all in one place for a while.
So yes it is reasonable to believe. The size of this continent can make a bit of a difference on climate depending on how big it really is. The farther away from oceans and other large bodies of water, the drier the interior will be and the more extreme the seasons too, much hotter in the summer and colder in the winter.