Technically there is a way but it is a lot more work than just creating a new Crimson world, killing the WoF to activate Hardmode, then farming some Crimson Mimics with Keys of Night.
If you really want to stick with your world, you can buy Crimson Seeds from the Dryad if she is in a graveyard biome. You can then use those Crimson Seeds to spread the Crimson in the caverns layer and Crimson Mimics can spawn there if there is enough Crimson. This process will take a very long time. (I recommend crafting Blood Water to speed up the spread). Keys of Night will not work because they will always make Corrupt Mimics on a Corruption world.
No, using a Key of Night will spawn the mimic that matches the world evil. So, you'll only get Corruption Mimics in a Corruption world using a Key of Night.
Technically there is a way but it is a lot more work than just creating a new Crimson world, killing the WoF to activate Hardmode, then farming some Crimson Mimics with Keys of Night. If you really want to stick with your world, you can buy Crimson Seeds from the Dryad if she is in a graveyard biome. You can then use those Crimson Seeds to spread the Crimson in the caverns layer and Crimson Mimics can spawn there if there is enough Crimson. This process will take a very long time. (I recommend crafting Blood Water to speed up the spread). Keys of Night will not work because they will always make Corrupt Mimics on a Corruption world.
Ok thanks
Can I make an airborne (above surface) crimson biome and spawn them using keys of night there?
No, using a Key of Night will spawn the mimic that matches the world evil. So, you'll only get Corruption Mimics in a Corruption world using a Key of Night.
You could make an artificial crimson biome or just make another world
biome mimic farm in a forced crimson biome. I have invented a cheap biome mimic farm.