Maddison pulled out of the squad to face the Czech Republic and Bulgaria last month because of illness but the uncapped 22-year-old was pictured in a casino during the 2-1 defeat in Prague.
Asked whether Manchester City full-back Walker's England career was over after amassing 48 caps, Southgate replied: "Absolutely not".
Liverpool's Alex Oxlade-Chamberlain has also been brought back into the fold after a long period out of action, having contributed four goals in 12 matches for Jurgen Klopp's side at the start of the new season.
"Now he will realise, as an England player, there is a different profile".
"We've been very pleased with the form of Kieran and Trent", said the England boss.
"He's very close, he's just got to keep on playing the way he is, and if we had issues over the weekend, I wouldn't hesitate to call him into the squad".
England lead Group A with 15 points and need one more point to secure their place at Euro 2020.
"And it's the 1,000th England game - a momentous occasion for everybody and a pretty special to be involved with".
But Mason Mount is now a doubt after suffering an ankle injury in Chelsea's 4-4 Champions League draw with Ajax on Tuesday.
"As a manager, you really get a feel of the history and the privilege of leading the team, when so few people have had the opportunity to do that".