They seem to be thin on the ground so phono to USB looks more do-able.
http://www.richersounds.com/product/turntable-accessories/project/phono-box-usb-v/proj-phonobox-usb-va
Leads me to think you may be better off buying an already USB-capable turntable.
I'm not familiar with how you "mount" these USB devices on your HiFi. I'd presume plug and play.
@mi7rennie started
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They seem to be thin on the ground so phono to USB looks more do-able.
http://www.richersounds.com/product/turntable-accessories/project/phono-box-usb-v/proj-phonobox-usb-va