Yes. For example, if I had the following scene...
...and I wanted connect the Button node's signal
"button_up" to the root node in the script attached to it (script.gd in this case), I would connect it like this:
onready var button = $HBoxContainer/Button
button.connect("button_up", self, "_on_button_up")
If you need to emit and connect signals from different scenes, it is a good idea to have EventBus singleton (here is a good tutorial).
I always connect signals in code but there's no right or wrong way here. But I think that it would be extremely hard to only connect signals through the Editor so it's likely you will need to connect signals in code anyway so I think it makes sense to always do it in code just to be consistent (of course, this is just my preference).