I don't have overall numbers, but I do have at least one good example:
Desde la Secretaría de Seguridad Pública municipal emergió José Daniel Rivera García, (a) “Zafiro”, sicario de “Los Zetas”, quien se mantuvo asignado en la Zona Hotelera como encargado de los operativos y patrullajes policíacos. Ahora se sabe su doble función porque él es uno de los 9 sanguinarios asesinos detenidos y presos en la cárcel pública municipal.
De policía a sicario y sus jefes hacen como si no supieran
A rough translation is:
From the municipal Ministry of Public Security emerged José Daniel Rivera García, aka "Zafiro", hitman of "Los Zetas", who remained assigned to the Hotel Zone as the person in charge of police operations and patrols. Now he knows his double role because he is one of the 9 bloodthirsty killers arrested and imprisoned in the municipal public jail.
From police to sicario and his bosses act like they did not know it
This example is given in the paper Who becomes a sicario and why? A supply-side analysis of Mexican hitmen (also: download link), which has some other possible contenders although I have not been able to verify any of them myself.
The book El Sicario: The Autobiography of a Mexican Assassin says that people are being recruited from the police academy:
Of these two hundred graduates [of the police academy], fifty are already on the payroll of the narcotrafficking organizations.
The author says that these 50 are broken down into several groups, including one that actually does the executions (p79).
The author goes on to mention the specifics of this corruption: "official police vehicles" are used to transport drugs (p76) and police cars are used to follow people who are to "be kidnapped or disappeared" (p80).
Unfortunately, I'm not sure if the author was working for the cartel and the police simultaneously since I'm only going by what I can see in the Google Books preview.