)A
I am trying to interface a 4-20mA two wire pressure transmitter ( loop powered ) to a USB data logger. The transmitter needs 9-32V excitation. I an trying to get this from the USB 5V dc and step it up to 15V with a Murata convertor (MEV3S0515SC).

The transmitter needs 5mA so with 15V this will be 0.075W.

that is not possible, a 4-20mA transmitter must, as a minimum, need 20mA. Thus I GUESS (note 'guess') the 5mA is what the transmitter needs in addition to the transmitted current. thus you will need 25mA.

)B
now reading your post in a different way
I am trying to interface a 4-20mA two wire pressure transmitter ( loop powered )
if that is so, you need no USB power.

)C
now it could be that what you are saying is that you are making a USB device that connects to/'reads' a loop powered pressure transmitter in which case the currents in )A apply

Erik