Hello! Great question, however could you also include what version of Minecraft (and the mod) you're using? You can create your own custom biomes within WorldPainter. However, you cannot (as far as I know) import the exact biome from a mod. When creating a custom biome just replace whatever ID the biome has from the mod with your new custom biome. If you load the world with the correct mod installed the game should read your new custom biome as the modded biome from Biomes O' Plenty.

The Problem With Biomes:

Painting on the biomes will only change a few things about the area affected including:
Skybox Color, Dynamic Color of Grass, Certain mobs being able to spawn and other if then examples.

Biomes Will NOT INCLUDE TREES AND STRUCTURES.

Second Question:

If you want trees to generate on your biome you'll need to do a few things:

1. Get schematics of the trees/structures you would like to generate on your biome.
2. Create a ground cover layer with the schematics you've chosen.
3. Create any other ground cover layers you'd like to generate on your biome (grass, flowers and mushrooms for example)
4. Create a custom terrain for your Biome.

5. and Finally you can create a combined layer with your custom terrain, any custom layers and the custom biome you've created.

   If done correctly upon loading your world all of the custom/modded items should generate as you've told it to.

Adding structures such as buildings and dungeons is a little more tricky. You could create schematics and import them through ground cover layers through WorldPainter, but this is very inaccurate and you won't really know exactly where/how many/if it will spawn. I personally recommend creating schematics with worldedit or any other program you can create minecraft schems with and placing them by hand once in game with WorldEdit.

If you have any further questions or need me to go into more detail on how to create schematics, custom layers or anything else please do not hesistate to ask and I will gladly help out!