Adding Custom Blocks to Worldpainter like Bamboo is a bit tricky, since its not directly within WorldPainter Embedded.
So here's what you need to do:

Use with WorldEdit

1. Place multiple Bamboo
2. Let them grow to different heights
3. Select each Bamboo separately (1x1xheight)
4. //copy (just stand infront ofit)
5. //schem save Bamboo_1
6. Your file will be in your server folder under /plugins/WorldEdit/schematics/
7. In Worldpainter go to Layers > "+"-Sign > Add custom object layer
8. Add the .schem file from WorldEdit.
9. Paint in your new Bamboo or combine it with others
   

Use with Structure Blocks (no mods required)

1. Place Multiple Bamboo
2. Let them Grow to different heights
3. Get a structure_block with the command: /give McMeddon structure_block 1
4. Select the "save"-mode with the (​left button)
5. insert your dimensions of the bamboo and relative position
6. Name your File & Press the (​right) "Save"-Button
7. Your NBT file will be in your WorldFolder under /saves/world/generated/
8. In Worldpainter go to Layers > "+"-Sign > Add custom object layer
9. Paint in your new Bamboo or combine it with others

Tipp: you can also generate folder structures with WorldEdit and StructureBlocks to sort your creations

Filename Name:     minecraft:plants/bamboo_1
Structure:        minecraft:folder/filename