I'm afraid not. Eurostar's ticketing rules forbid this. From their FAQ:
Can I board the Eurostar from a different station?
For journeys from the Continent to the UK, you will only be able to board the train from the station indicated on your ticket.
Starting from the UK, if you have a ticket from London, and your train calls at Ebbsfleet or Ashford International, you can board there instead. Conversely, some tickets nominally Ebbsfleet or Ashford allow boarding at London, but not all of them, check the fine print.
You can leave earlier if you want. This is official if you have a ticket to London and want to step off at Ebbsfleet or Ashford; on the continent side, since no one checks disembarking passengers, you can in practice.