Voluntas comes from the verb volo. Roman verb-derived names are fairly common. For instance, from valeo we have Valens and Valentinus; from vinco we have Victor and Victorius.
From volo I would derive Volens, Volentinus, or Voluntarius. Of those three, only Voluntarius really seems to derive immediately from the noun form.
For unfamiliar words, it never hurts to check the dictionary. Here is what Lewis and Short have:
Īō, Iūs, and Īōn , Iōnis, f., = Ἰώ, I.a daughter of Inachus, king of Argos, beloved by Jupiter, and changed, through fear of Juno, into a cow; afterwards worshipped as an Egyptian deity, under the name of Isis. —Form Io, Ov. H. 14, 85; Prop. 2, 28 (3, 24), 17; ...
A number of Greek names ending in -ō have both Latin and Greek style declensions.
One would indeed expect the typical Latin form Ionem, but Io is valid as well.
The context is fortunately clear enough so that we can be confident that Io is ...