Upvote:3
Unfortunately no, it does not seem possible to update your ticket with a membership card number after buying the ticket.
On the help of voyages-sncf.com{french}, the question is answered with the suggestion to exchange your current ticket for a new one, mentioning your membership number. My opinion is that it is worth only if you can cancel your ticket for free (ticket "Loisirs" and "Loisirs Jeune" and "Pro" are usually cancellable without a fee) and get a new one for the same price or a very close price. So you can check the current price of the ticket you would like to exchange and figure if this is worth it.
Note that after the trip, it is not possible at all to associate this number to your ticket.