That's one thing.

The second thing, you can do an automatic mapping. Of course you can. Everything depends on what mapping you prefer. Random or "Authentic".

Random mapping is relatively easy for every mode, but it produces garbage tier maps, that can be only generated for fun or practice, like I did with my Taiko Map Generator and osu!catch Map Generator. I haven't tried Mania and standard yet, but that should not be hard.

Why random mapping is easy?
osu!
Well, for osu!, you just pick random starting X/Y, a direction for 2D vector, and do whatever you want. Playing with the coords by adding a value in distance to X/Y and reversing the value when it goes off-screen will result in a straight line mirroring from the playfield edge. You can also swap HitCircle for a linear slider (they are easy to code).
The most complicated thing for osu! is making curved patterns I guess. You can pick another random X/Y coords, and create a curve from your current location and the new point, then either calculate how many notes you can insert between these two points in time or make a slider from it.
When you mix everything together, you can get something like early-2009 mapping (?)

Taiko
For Taiko, there is nothing much to explain. You just insert a don or kat according to your bpm and snapping divisor.

osu!catch
It's a bit a pain to code, because of the off-screen checks, but everything depends just on setting the next point in distance from your current X to another. I made it by dividing the distances between normal movement, dash and hyperdash as the following
normal: 0 - 64 units
dash: 96 - 144 units
hyperdash: 160 - 256 units (that's because 256 units is the half of game playfield, so bigger hyperdash would spawn next note off-screen when the current location is in the middle)
next_note + (calculated_distance * direction) where the direction is either 1 or -1

Nothing special unless you add sliders (which were a bit harder to implement. I have some example maps of course).

osu!mania
Random mania mapping would be interesting, like dividing the generation in few steps. Fill the map with some Holds, then try filling the map with single notes (with checking the current spot and column for any collisions like if there is a Hold in the place where you want to add a note). You could also check if there's a Hold's end at the current spot so you can place a note there, and repeat for multiple column usage. Might be complicated a bit I guess.

That's for Random mapping. What about Authentic mapping?
Well... as people already said, nothing will replace human when it comes to mapping. You can only create something a bit closer unless you spend enough time to understand how people make their maps. I'm not saying it's impossible, because you can make pretty much everything if you spend a lot of time on researching and on lots of Trials and Errors.