You can bake (select voxel world and go down to Voxel - Bake) the voxel world out to be a normal landscape mesh then use the landscape foliage painter tools from there.
If you want to make instanced foliage native to Voxel Plugin you either need to code your own HISM or get Voxel Plugin Pro.
If you decide to go with Pro I made a tutorial that talks about this stuff.
