Passing extra vertex data to a shader

In UDK there is a thing called "Dynamic Parameter" for particles which acts as a second
vertex color: It's generated by the particle system and passed down to the vertex and fragment
shaders where it can be used freely. Can I do something similar in Unity? I'm generating a mesh
with this extra vertex data but I'm not sure how to pass it to a custom shader. ShaderLab's
appdata_full seems to have a fixed set of parameters.