That's nothing to do with FRED, and shouldn't be - FRED doesn't get to fiddle with any other ship properties, so why would this one be different?
This kind of adjustment is what modular tables are for - if you want the 'same' ship to behave differently in different missions, then you make ship clones like SF Dragon#weak.
(Yes, you may start to hit the ship limit, but it's pretty high)
FREDding to auto-repair the hull is pretty easy anyway - just use a repeating When with a suitable repeat rate and very large repeat count, and repair-subsystem > hull each time
Using an argument list makes it even easier, as you just list the relevant ships that self-repair as the arguments.
(Although you'll need to understand arguments first. And those *are* more complicated.)