Fisher, who turned 32 earlier this month, joined the Broncos as assistant coach at the end of last season but the club took the precaution of lodging his playing registration with the Rugby Football League.
"It was always part of our plans, it was an option from day one," said London head coach Tony Rea. "He's looked sharp in training this week."
Fisher, an Australian-born Scotland international, made 124 Super League appearances for Hull KR from 2007-11 and made 14 appearances for Catalan Dragons last year.
The Broncos are one of only two clubs yet to win a match after the first three rounds of Super League.
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