Which book has the stats for those medical frigates?
Is there no demand for more spaceship deckplans?
The Nebulon-B has been used as a medical frigate, and it's in the AoR core. Other ships can be used in a similar way, and dedicated conversions--such as the CR90 becoming the Cybershop Ship from Special Modifications --are possible but not always necessary. The simple fact is that the real doctors should be where the concentration of Alliance personnel is highest (larger bases and capital ships).