seems to be some confusion here, let me explain the best i could,
i have a e90 canadian and had done tire/wheel changes.
BMW calls a silver valve stem thing on the oem wheels TPMS, while BMW Canada calls the on board computer + ABS tire pressuring monitoring system TMPS, which is why its a little confusing.
US Cars- have TPMS, it is a silver stud thing on the valve stem, this silver stud thing measures tire pressure. here is a picture, and CANADIAN SPEC E90's don't have this.
for US Spec E90's if you want to change rims/tires, you will have to buy new TPMS studs from the wheelshop or just use the existing OEM TPMS.
Canadian Spec E90's don't have this TPMS on the valve stem, they are just normal standard black color valve stems. Canadian spec E90's using an ABS monitoring system to check for low pressure.
So if you are to change tires/wheels, all you need to buy are wheels + tires, no individual silver TPMS, but make sure the rolling diameters of the wheel+tire is the same or it will throw errors. The E90's OBC on board computer calls this ABS monitoring system low pressure tire monitor system, "TMPS" or something like that, which could be confusing.
So OP, go ahead and change your oem wheels to steelies, make sure the rolling diameter is the same as oem. if the obc computer throws errors, reset the pressure monitoring system on the computer and let it adapt the new diameter as the standard. and you are good to go.