Welcome!

If you created the Custom Profile Property on the Manage Profile Properties page, yes. That is the right way to do it.

As for populating the list from code, you will find the list items in the Lists table in your database. If you want to populate the list from a database table that you have, you will need to write SQL to add the data from your table to the Lists table, and in the correct format. If you look at the Lists table, then you shoule be able to understand what you need to add.