Sourcegraph Director of Product here: We currently don't support installing extensions in our UI, but it's something we're definitely interested in. I'd love to hear more about other use cases you know you'd personally like to use?

You can however set up your own keybindings by editing the keyboard shortcut settings. Open the Quick Open (on a Mac, Command+Shift+P or F1 on a PC), and type "Open Keyboard Shortcuts". From here you can configure any mappings you'd like.