You can’t currently do it at Nîmes ticket office, only at a RENFE ticket office in Spain.

But there are recent reports of being able to pay onboard the train to the ticket inspector.  This is very sub-optimal, as you can’t be sure until you’re on the train!

As long as there are seats available (check on renfe.com), you can speak with a ticket inspector before boarding and then pay the €10 on board.

I travelled on this train yesterday and was told this is indeed the case. If full, they suggested they'd even be willing to allow passholders to travel in the bar coach (though that's not very comfortable: it'd be standing as there are no seats in the bar).