It's not. PL/SQL runs on the database server. It has no access to user input whether inside or outside of a loop.
You can write a SQL*Plus script that includes 10 substitution variables, prompts the user for values, builds a PL/SQL block that computes the sum, and then submits the PL/SQL block to the database to be executed. That will only work, though, if the caller is an application like SQL*Plus that has the ability to use substitution variables.