I'd be surprised if your prox sensors have built in pullup resistors. I'm using 2.2k pullups on prox sensors in my 24 volt system on the manufacturer's recommendation..
You will need something to convert voltages. There are some signal chip solutions that do the trick. I looked on another forum for the part as it was mentioned recently but couldn't find it. Maybe you could use something like this which I found on a quick Google
https://www.automationtechnologiesin...face-breakout/