Options here...
Cannibalize Hull:
Sacrifice Hull HP to boost armor. Crew starts taking parts of the ship and slapping them on as armor in an emergency. This is tank mode. All alone under fire, hit the tool. Start losing perma hull and gain armor. Direct tradeoff. So if the armor drops, you die easier, but it'll be like a turtle till then. Also since it sacrifices hull, it could leave you weak for future engagements. Kind of a...oh crap, meat grind, tank! Tool.
Now should it be able to kill a ship? *shrug* If not kill, leave it at 1% and make the tool unable to be used after that.
Now as for how much dmg would overwhelm it? Dunno, talk it out.
Cannibalize Armor:
Sacrifice armor to reinforce balloon. Crew starts tearing off the armor and slapping it on the balloon. Replenishes HP. Same effect as the hull cept armor can be rebuilt. Possibly give it faster effect to make sure the armor will go down quickly.
Could be possible to make it a direct mirror of the other and just code it so the armor doesn't come back. Flying around with your balloon hull ship.
Cannibalize Balloon:
Either Hull or Armor gained by sacrificing the balloon. Crew adds on armor/hull at the price of losing capacity on the balloon. This one might need a cut off point, otherwise you could suicide pretty easily.
Cannibalize Armor 2:
If armor could be coded to be completely removed with a tool, or maybe brought down real low. Similar trick to engines or weight of the vessel. Crew sheds armor to gain more speed/mobility.
Unfortunately all of these are pretty much 1 time use tools and situational. But each has drawbacks to usage. Someone using one, would not be able to sustain it and it would leave them weakened for future engagements.