I've never had to go that far with our
Trillium - hopefully others that did will chime-in - but in ours there's a long vertical rounded piece of
fiberglass on the inside (behind the ensolite) that covers whatever wood may be behind it. So I suspect you'd have to cut out that
fiberglass piece (or at least the sections over the hinges) to get to the wood, and then glass it back on somehow. Seems like a lot of work to me, but best thing to do might be to peel off a section of the ensolite near the door and evaluate. An alternative approach might be to mix some fiberglass resin, and use a big syringe or straw to push a big glob of resin inside the hole, then fill the hole itself, let it cure, and drill into it.
One thing worth noting is that you'll probably want to realign the door a bit before making new holes to correct any sagging.