PF: A researcher who is currently employed with an institution in country A would like to apply with that institution for the Postdoctoral Fellowships call. Is that possible?

The mobility rule is clear - the researcher cannot have resided or carried out their main activity (work, studies, etc.) in the country of the beneficiary for more than 12 months in the 36 months immediately prior to the call deadline. If the researcher respects this rule, it should not be a problem that the researcher is already working at the future host organisation.