not only ... (but also); not just; as well as
Used to add information, often followed by ～も or ～さえ. Connects nouns, verbs, and adjectives.
He can speak not only Japanese but also English.
This store is not only cheap, but the quality is also good.
だけでなく is the neutral, everyday way to say 'not only X' while ばかりか adds a stronger, often surprised or emphatic nuance to the additional element.
だけでなく means 'not only... but also' and is more colloquial, often used for contrast, while に加えて is neutral and simply adds an extra item.
Compound of だけ (only) + でなく (negative continuative of だ). The exact historical derivation is straightforward.