Connect a Lacie Thunderbolt only drive to a USB port?
Thunderbolt is PCI express, so you would need an older machine to have a PCI express bus or card and then add an adapter card which would cost far more than a new USB drive.

I don't think that adapter (which is eminently makable) would be brought to market due to the cost of the "solution" to be implemented.

If you need a drive that connects to old and new computers, perhaps trading your awesome Thunderbolt drive for a USB one would make sense, but you might instead put the files on an internet share to do short term sharing of the data.