Summary: Bitcoin's Lightning Network cannot be added to a Metamask Wallet because it is not an Ethereum Virtual Machine (EVM) compatible chain.
To use the Lightning Network, users must download a Lightning-enabled wallet like Bitcoin Lightning Wallet (BLW) or Wallet of Satoshi.
Can I add Bitcoin Lightning Network to Metamask?
Metamask does not currently support the Bitcoin Lightning Network because it is not built with the Ethereum Virtual Machine (EVM) native programming language. The Metamask wallet only provides support for EVM-compatible networks that are built with the Solidity programming language.
What is the best Lightning Network wallet?
The best and most popular Lightning Network wallet for Bitcoin is Wallet of Satoshi. It allows iOS/Android mobile users to store, send, and receive Bitcoin (BTC) via the lightning payment channels. The Wallet of Satoshi app has completed over 3.5 million transactions over the lightning network, making it one of the most popular crypto wallets.

The Wallet of Satoshi also offers a convenient in-app way to buy Bitcoin with a Debit Card or Credit Card. This means you don't have to worry about transferring BTC to a new lightning network wallet if you are just looking to test out the infrastructure and hype underpinning the lightning ecosystem.
Decentralized Finance (DeFi) on Bitcoin Lightning
As at time of writing, there are no decentralized finance applications on Bitcoin's Lightning Network that allow you to earn yields on your BTC. The only real use case for Lightning Network is lightning payments, which allow you to transfer BTC in a peer-to-peer manner with low fees.
About Lightning Network
Lightning Network is a "Layer 2" payment protocol that operates on top of the Bitcoin blockchain. It enables fast, cheap, scalable transactions between and across participating nodes. Lightning Network was first proposed by Joseph Poon and Tadge Dryja in a 2015 white paper. In 2017, Lightning Labs released the first implemention of the Lightning Network.
Lightning Network is designed to solve the problem of scalability in Bitcoin (and other blockchain-based cryptocurrencies). Transactions on the Lightning Network are fast and cheap because they are not recorded on the blockchain directly. Instead, they are only finalized when both parties agree to commit the transaction to the blockchain. This "off-chain" approach reduces the load on the blockchain, allowing it to scale to handle more transactions. Lightning has the potential to scale Bitcoin to handle millions of transactions per second. It is a key part of the Bitcoin roadmap and is being actively developed by a number of companies and organizations.

Final Thoughts
In conclusion, the Lightning Network is a promising solution for scalability in the Bitcoin network, allowing for fast and cheap transactions. However, it cannot be added to a Metamask wallet because it is not built with the Ethereum Virtual Machine programming language. Users must use a Lightning-enabled wallet such as Wallet of Satoshi to access the network. Although there are currently no decentralized finance applications on the Lightning Network, it is still an important development in the growth of the Bitcoin ecosystem.