We already know what the medusae powers are, they are not psyker powers like the farseers. Purely from the fluff point of side, this can't be justified, a medusae could ride a bike, if the thing infected a reaver somehow, but it wouldn't get powers like those of a farseer. A medusae isn't even a psyker to begin with, though it does have somekind of weird powers. A medusae does have the power from pain, it still counts as dark eldar in theory, though it's a parasite infecting a dark eldar.
A medusae somewhat half infecting a farseer, but not completely, could represent this, but it's still not 100% correct fluffwise. You could simply change the name of the parasite to make it your own, using the medusae model, some kind of parasite with farseer powers. Or it could simply be a corsair psyker, very little is know about corsair psykers, though forgeworld has looked into this, but options are still open.