Hey there, JaiHazel.
What's the model of the laptop? Does it have a 2.5" drive bay, or an optical drive? Basically those are your options to connect an HDD if you want to expand your storage capacity. However, using the optical drive option would require to remove it and put an HDD/SSD caddy in its place (basically a removable 2.5" drive bay).
Also, what do you mean by "default storage"? Is it that you want to move your user folders over to the HDD? If yes, will you please let me know what's the Windows version you're currently using.
Boogieman_WD