Any reason not to just use a phone to write to it?
NDEF is simply an “envelope” around data that you store on a tag. You don’t convert a tag to NDEF but you can write NDEF records to it. The NFC tools app can do this very easily.
Did you buy the PN532 from DT or from somewhere else? The reason I ask is because there are fake ones out there and they have VERY poor coupling performance to the xNT.