There are a couple of options available but they all use the same method.
My plugin Project Worlds has a fill command that will pre generate chunks in a defined radius around spawn
Also my other plugin Project Borders can generate chunks within the configured border
There is also a chunk pre generate plugin somewhere but can’t seem to find it.
Results seem to vary from server to server, You can run it live however I wouldn’t recommend running this with a bunch of players logged in. There’s been reports of issues with chunks not unloading when generating, causing a great deal of Entity lag and possible server crashes.