I haven't found this option at the weapon crafting station, which was my first thought, and a brief google shows Fallout: New Vegas had it, so I'd think FO:4 would have it too. I don't see it anywhere though. If there's no way to craft ammo, what's the best way of gaining/hoarding lots of ammo?
You can craft ammo with the Contraptions Workshop DLC using the Ammunition Plant.
I have yet to try this in-game myself, so this answer isn't very detailed, but I'm surprised no one has written a nice answer about it yet, so let me at least note it.
The only special ammo in fallout 4 you can create is for the syringer rifle.
However, the shops and the wandering caravans tend to hold a lot of ammo.
Random caravans will show up in your settlements, such as sanctuary. Also the weapon dealers in Diamond City and Good neighbor tend to carry a lot of ammo.
Finally there is the scrounger perk that lets you find more ammo as loot.
If you build the ammo maker in the workshop, you can make most of the ballistic ammo like .38, .308, .44, .45, and shotgun shells. If you have all the levels of the gun nut perk, you can put the components in the machine and then tell the computer you have to hook up to it what kind you want to make. Most need fertilizer and lead.