The 25-year-old initially signed for the Rams on a month-long deal, but he will remain at Pride Park until the end of December.
Stoor was set for a return to London after Saturday's 4-2 defeat to QPR, where he was substituted after picking up a black eye.
"It is a compliment that Freddie wants to stay," manager Nigel Clough told the Derby Evening Telegraph.
The former Rosenborg man will look to add to four appearances picked up during his initial spell.
Meanwhile, the Sweden international is wanted by top flight clubs in Greece and Turkey, according to his agent.
"Clubs in the top flight in Greece and Turkey are interested," Per Jonsson told Fotbollsexpressen.
"At the same time, Derby want to loan him until the end of the season. Fulham have been to watch him and are pleased with what they have seen."