There is no syntax for that. While a regex can be used to match all of them, it won't really help much with replacement – that in itself is already outside of what a regex does; there is no regex syntax to specify what which match should be replaced with. The program that does the regex match still needs to "manually" map each match to its replacement.
In short, it depends on the multi-rename tool and not on the regex.
If this were on Linux, I would use perl-rename (aka prename) as it's arbitrarily programmable, though in this case the simplest option of chaining multiple regular substitutions would do the job. (Perl also has a character substitution operator y/// that looks like regex but isn't.)
prename "s/Ά/Α/g; s/Έ/Ε/g; s/Ή/Η/g; s/Ί/Ι/g; s/Ό/Ο/g; s/Ύ/Υ/g; s/Ώ/Ω/g" ./Docs/*.pdf
prename "%r = ('Ά' => 'Α', 'Έ' => 'Ε', ...); s/[ΆΈ...]/\$r{\$&}/ge" ./Docs/*.pdf