I don't know how many people who are on repl.it play Minecraft let alone create spigot/bukkit plugins for servers. In order to make a plugin for a Minecraft server, in Eclipse (Java IDE) you right-click the java project you're working on and choose "Add External Archives" this allows you to add the spigot API from the spigot-VERSION.jar
I was wondering if there was any way I could do this with a java repl. If not, it's alright, I was just wondering.
you can import external packages like in any other language on repl. Package support and availability may vary, but in general if you just require it at the top of any repl, repl.it will automatically add the package. You can also add the package manually; there's a set of controls on the left hand side of the repl.