yes get your external hhd conected via usb then what i would do is trancfer as much as you can to the external drie eg pictures documents music ect so you only have your system and software on you internal drive
OR you could install a new biger hhd then trancefir the stuff you wont back onto new hhd
Sure you can add an external hard drive. You can use it to store whatever you'd like and free up some space on your laptop's internal drive. Remember though, you cannot run an OS from an external drive.