There is no reason not to be able to do that in Honeybee. Honeybee accepts EnergyPlus materials and constructions as string (text) so as far as you have the right definition it should work. Also read my reply here: http://www.grasshopper3d.com/group/ladybug/forum/topics/pcm-materia…
To do this, you will have to use the additionaStrings input of the simulation component. You want to add a MaterialProperty:PhaseChange object to the EnergyPlus file using these additionaStrings similar to what is done in this example that adds an evaporative cooling tower through additionalSrtrings:
Your MaterialProperty:PhaseChange object will have to reference the name of a material that you have assigned to a HBObject using the Honeybee components. Here is some more information on how the text of the MaterialProperty:PhaseChange object should be input:
The rest of the error message probably has some information about why that IDF string is not considered valid by EnergyPlus. You should also read the report in a case like that.