No, that radio module probably does not have aux input. That was one generation prior to the first confirmed part number with aux, and it was discontinued in 2002. I would not buy it unless you could confirm prior to purchase that the original owner used it with Aux. Here is a post that lists the bord monitor radio part numbers and their known features. If you get one of the newer radios, then yes, you can add aux to your car.

BM53 Radio Module Part Numbers

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input