On testnet, mainnet addresses are basically "aliases" of their equivalent testnet addresses. Sending to 1KNHQ7513gVwJv5fG4ucsPgkdb5Ub9UzR6 is exactly the same as sending to mytEhA9yrhwC62ZGydszhJu5VagBZj2z7t. The only difference between these two addresses is the version and checksum: the public key hash is the same.

If you copy your wallet.dat from mainnet to testnet, your receiving addresses will all still be there: they will just have been changed to their "testnet" aliases. The associated mainnet versions will still work for transactions on testnet.

On testnet, there are actually 111 valid ways of expressing of every address. These are all the same:
1KNHQ7513gVwJv5fG4ucsPgkdb5Ub9UzR6
ihtPDNHkrxp8MDkHVEwMWxYG6LRMEvGhC
283VNKfaU3RgwnMqJuaFqeEKtbbN1Q5KFY
2XP6MRxsBDtZmDVvLKuaKmW7X6rJiSaZ9Z
2vihLYG9tQMSaee1MkEtotmu9c7FSBxg9a
3L4JKeZSbapKQ5n6PAaDJ23gn7NCD2x9V1
...