I have never found a Blood Bowl conversion. Primarily, this stems from a couple of reasons as far as I can tell: 1) the teams in context of the WFRP world make no sense and 2) the game too closely resembles the NFL, i.e., American Football.
That said there is the game of Snotball as described in WFRP 1e (first edition) Middenheim: City of the White Wolf. On p. 90, the game is described as follows:
Snotball (Middenheim Rules Version) is a City craze and some people come to the City just to follow this ancient sport. Unfortunately, Snotball matches do attract many Football Hooligans and violent confrontations between fans of rival teams are not uncommon.
Already you see the resemblance to football as the British know it... "Go Man U!" (Mind you I don't really have an affiliation in any league and only barely follow the Premier League because of my employer.)
In brief, the game is played between two teams of 11 players, on a pitch rougly 200 feet long by 85 feet wide. There is a set of goals at each end of the pitch, both 11 feet wide and 7 feet high.
The snotlings used are fairly young, and must measure between 18 inches and 21 inches so they aren't too heavy and can be booted about easily.
The Snotling is anaesthetized with a curare-like paralyzant; not for its benefit, but because otherwise it might be able to bite and scratch if the leather straps worked loose. Snotball leather-workers are very proud of the craftsmanship, however, and this rarely happens.
Ha! And humane!
Players wear extremely thick, brightly coloured leather jerkins, tousers, shin padding, and a mail coif.
Equipment for a player! A starting kit for a career.
The game is played over two halves of 30 minutes each, with a 15 minute interval at half-time. Two additional periods, of 10 minutes each, are played if the scores are level at full time. Two substitutes per team are permitted.
The aim of the game is to get a Snotling (which is contorted by very tough leather thongs into a roughly spherical shape) into the opposition's goal by any means the players can come up with (kicking, throwing, punching, nutting). Just about the only rule is that no part of the attacker's anatomy my be in contact with the Snotling as it crosses the goal-line. So no rugby-style scrum diving is allowed in scoring a goal (although it can be used to get the Snotling very close to the goal).
Some basic game structure... and that's it.
My initial recommendation for a career: re-skin the Pit Fighter and Judicial Champion for Basic and Advanced career paths, add fluff. I might limit actions to move and feints with the grapple rules and Strength Bonus for kicking "movement" using the SB as the Move attribute calculation.
Marc Torley on G+ suggests another perfect template for a Snotling Football player
BB is a way to follow a team in a sporting event. Snotball is more an excuse to beat up your neighbours. The best template for a snotball player? Thug. Add Flee! And it's ideal!
Have to agree, then you get the career path of Thug.