Ritchie, who scored in Monday's 4-2 victory over Charlton, has been told he is surplus to requirements at parent club Portsmouth.
And after a successful three-month loan spell, Fitton has confirmed the winger is set to sign a two-and-a-half-year deal at the County Ground in the next 48 hours.
"This transfer is more complicated than normal from a paperwork point of view," he told the Swindon Advertiser.
"There is no problem but there are new procedures we need to go through. The paperwork is a bit more complicated but I am confident (Ritchie will be signed before the Oldham fixture).
Fitton also confirmed the club have not received a bid from Birmingham for star striker Charlie Austin.
Source: PA
Source: PA