pre-drilled would have been the way to go, but it's not an impossible thing to do. The key is to not try to just drill into it. You need a pilot hole to break through the surface and then carefully enlarge the hole to the size you need. The biggest concern is chipping the finish next to the hole. Depending on the style of knocker used, the knocker itself may provide some cover outboard of the attachment holes, and allow for some slop but still have a nice finished product at the end.

a quick google search for "driling a fiberglass door" led to the attached link which seems to provide a reasonable approach on how to do it.

Good luck.

Here is a link that might be useful: drilling a fiberglass door