These remotes are top-end learning universals. They can be taught IR codes from a donor remote. This is done the same as all other learning remotes; by pointing the donor and pronto remote head to head and using the learn function.
The vast majority of product manufacturers use standard IR frequency codes that are learnable. The notable exceptions are B&O and NTL.
Grundig have been using standard learnable IR codes for a long time. I won't say forever because there's always some smart arse who pops out of the woodwork to say that an obscure model used laser beams or telepathy or something equally bizarre.

Let's say it's a reasonably safe bet that the Grundig can be controlled by the Pronto.
I've also cross checked with the URC and UEI code databases. There don't appear to be any exception notes about the Freesat box you own. In otherwords it appears compatible with the the OneForAll type universal remotes which means it's very likely a standard IR code set.
One can never be absolutely certain, but as educated guesses go I'd say we're on pretty solid ground.