Generally the tags should follow snake-case not camel-case, so it should be ref:kep_id. Because this id is something country specific and to not conflict with something else used internationally, I suggested to namespace it with the country code using ref:GR.

Some other suggestion is that the building tag is mainly used in ways and not in nodes. If you see the building tag stats (https://taginfo.openstreetmap.org/keys/building#overview), there are 592 million ways with the building tag and only 1 million in nodes. Also the tag description in the wiki (https://wiki.openstreetmap.org/wiki/Tag:building%3Dgovernment)  mentions "Values of the building=* key should reflect the construction of the building, not its function". So it would be better to tag with office=government and remove the building tag (unless your tag a closed way, in that case it should be building=office).

Another thing that can be improved is that the name should not be all caps. There is an error showing up in Osmose for it ex. https://osmose.openstreetmap.fr/en/issue/d8017990-4dc5-d179-a12c-c32df15b2a88 (Name with uppercase. Concerns tag: `name=ΚΕΠ ΛΕΥΚΙΜΜΑΙΩΝ`). So I think it should be ex. "ΚΕΠ Λευκιμμαίων".