Can hackers add a malicious block to a DeFi staking network?

DeFi staking networks run on a blockchain themselves, so as long as the blockchain is safe, the blocks can't be malicious. Putting it simply, big networks like Ethereum, Bitcoin, Binance, XRP, and others are more or less "immune" to attacks. This is because they use complex consensus algorithms (both PoW and PoS) to validate the blocks. If the network is unreliable, then yes, attackers can definitely add a malicious block and destroy the network.