You can actually still reinforce them to some degree.
There is a special rule regarding supply and reinforcement which can help you to reinforce troops for which you cannot purchase ordinary replacements. It's most likely a legacy rule of older editions of the AGE engine.
Basically, you need a province with an exceedingly high supply value (in the 1st Punic War scenario, provinces such as Carthago or Lilybaeum). If you place your damaged troops there and if the province still generates excess supply after supplying those troops and all other elements in the province, the excess supply will be (seemingly randomly) used to heal damage.
In this way you can heal units that you cannot reinforce normally, but you can not recover completely lost elements. This means the Romans can still wear you down over the course of the war.