The 6ft 3in marksman, brother of Swindon midfielder Oliver Risser, arrives at the Recreation Ground as a free agent having most recently played in Malta.
Risser's arrival continues manager Dean Holdsworth's recent revamp of his squad and the 29-year-old could make his debut in Tuesday night's home clash against Hereford.
Holdsworth told the club's official website: "Wilko gives us something different for our strikers to play off.
"When I made a phone call to his agent, within 20 minutes Wilko was on a plane and made himself available. Currently he is on a short-term contract but he could become a good asset for us. He is well travelled and he wants to prove a few things to people and we can take advantage of that."
Shots have also confirmed the extension of defender Sonny Bradley's loan stay from Hull into a second month.