Hi fwu
Yes, that is possible from back-end.
Please follow the below steps to get custom social fields for user.
1)Go to WP Admin – Users – Profile Fields
2)Click on Profile Fields and click to Social Profiles.
3)Here, you can Click on "Add New Field" for your new social profile.
Refer:
http://prntscr.com/lngqyf
4)Note that the name of the social profile field label should match exactly with the name of the social
media website, for example for Facebook profile link, the label of the social profile field should be
“facebook” using the label, we detect the icon of the social site which is displayed on various locations.