Yes. There are factory 9mm SMG versions of the AR-15 and some mistakenly refer to carbine versions of the 5.56 AR family of weapons as “submachine guns.” However it would be unusual for police agencies to use semi auto guns modified to fully automatic. In fact, the reverse is true, it is common for U.S. police Agencies to converts surplus select fire guns to semi auto only.
There are many police agencies which use select fire versions of the AR-15/M16/M4 family of Weapons. The compact 5.56 versions are technically carbines because they use a rifle round though they are sometimes mistakenly called submachine guns. Submachine Guns, by definition use pistol rounds. There is an 9mm submachine gun version of the AR, the Colt 635 which has been used by police agencies including the DEA and the U.S. Dept Of State Diplomatic Security Service.
The original original Armalite AR-15s and the original “Colt Armalite AR-15”s were select fire, meaning they could fire either semi or full automatic. Colt introduced the semi auto only AR-15 in the 1960s after the U.S. Military adopted the select fire version as the M16 and later M16E1 /M16A1. In common use, AR-15 normally now denotes a semi auto weapon.