If you want to run a dedicated server, you need to build a server to run; which includes building from source.
If you just want to join a hosted game, as in another player is acting as the server, you do not need to build a server from source.
“Find Sessions” should return all multiplayer game sessions; hosted and dedicated. (for your game)
You can mix C++ and Blueprint. Most games do.
https://docs.unrealengine.com/latest/INT/Gameplay/ClassCreation/CodeAndBlueprints/
https://wiki.unrealengine.com/Blueprints,_Creating_C%2B%2B_Functions_as_new_Blueprint_Nodes