I have USD Fiat in my Ethereum MetaMask account. Is there a way to convert this to ETH?
This is really not about AGI. 
And no, you probably can’t just convert USD Fiat to Ethereum on Metamask. For that you’d have to log into a trading platform like Binance or Kucoin to exchange that USD fiat to ETH or whatever other coin you’d like to. Basically send that money to a trading platform and you can exchange it there for ETH. Hope that answered your question.
Also, monetary threads are not encouraged here.