I need data for 90 days while on holiday in france. Is it possible to pay for data after the 60 day

MSF is spot on.

If you have an esim compatible device, may well be worthwhile you looking at travel esims, such as "Holafly" or "Airalo", which offer reasonably priced data only packages for travellers. 

Another option of course is to purchase a French SIM card, which should be very cost effective for a 90 day stay.

https://www.expatica.com/fr/living/household/france-mobiles-sim-cards-1026216/ 

Hello Gillian_2003124 : The following article: https://help.smarty.co.uk/en/articles/5082043-roaming-fair-use-terms-and-call-rates-to-the-eu suggests that the 60 day limit will only be extended in extenuating circumstances. It also suggests that you contact Smarty to discuss those circumstances.

Somehow, I do not think that a holiday will cover it but always worth asking. Have you considered buying a local sim while there?