I transferred Ether to the main network in Metamask. This might seem like a stupid question, but how do I transfer that Ether to the Kovan Test Network?
You can't transfer ETH from one network to another. You can get free ETH on most testnets though, here's a link to information on faucets for the Kovan testnet.
There are currently four ways listed on the above link to get Kovan testnet ETH:
Icarus Faucet (SMS Verified, Automated)
Github Gist Faucet ( Automated )
Request Via Gitter (Manually Verified)
Use PoW via SmartPool