Hi,
"offline generating" of vouchers as such is not possible, pfSense needs to now somehow which vouchers are valid and which are not…
Maybe setting up a VPN to the pfSense-Box could help? Someone created a REST API for pfSense Vouchers (http://blog.digitalhigh.es/pfsense-voucher-rest-api/), you could build something on top of that, but you still need access to pfsense though…
Best regards (or "Viele Grüße" ;)),
Eagle2