I was wondering if it’s at all possible to add a backup hyperdrive to a ship which doesn’t have one (i.e., the stock version doesn’t have one). My ship-fu is extremely limited, so I thought that I’d ask the forums.
Can it be done as an attachment which uses hard points? If so, in which supplement can it be found?
Can it be done while crafting a starship based on a template/stock ship which normally doesn’t have a backup hyperdrive? Again, in which supplement can such an option be found?
As far as I am aware, there is no attachment for installing a backup hyperdrive.
Fully Operational has the rules for ship crafting, and I believe that it covers backup hyperdrives.
Assuming there aren’t any actual rules, I would just take the existing Hyperdrive Generator attachment as a base and just change the base modifiers to granting a backup hyperdrive at a rating of 15.
As for ships that don’t have a stock hyperdrive, I would have to be convinced to allow it, but would probably just increase the cost to 20,000 or something like that, changing the Hyperdrive Generator attachment base modifiers to granting a hyperdrive with a rating of 4.
Thanks for pointing me at Fully Operational. 
According the the starship crafting rules in that supplement, the hyperdrive is treated as a 1-hard point attachment with two different possible speed class ratings (depending upon how much one wants to pay). There are mods to that attachment which can improve the hyperdrive, put in a backup hyperdrive, and improve the backup hyperdrive. Since the hyperdrive is a pre-existing attachment, it looks like a backup hyperdrive mod could be installed in most cases (with enough cash, time, tools, skill, etc.).
That seems fair, though I would be inclined to at least take a hard point since the ship was built without a backup etc.
The idea with crafting is that you are building it into the ship. Once the ship is built, you can’t as easily* mod the base components.
*Speaking practically, not in game terms. In game terms, you flat-out can’t.