Additional Officer slots are coming to the C-Store in the near future - here's the quote from dstahl, as posted here:
Originally Posted by dstahl
Here's a few things I've seen on the C-Store dev schedule...
Bridge Officer Slots
2 New Ships from the ship poll (excelsior/nebula)
No dates on when they will all be available yet - but those items are in the works and will be here sooner than later.
What I'm guessing is that the tech to implement them has just been added, hence why the option popped up in the C-Store briefly. However, it's most likely disabled until the C-Store is officially updated.