I converted these toys (Plasma Catcher Eggs) into Tyranid Mycetic Spores around a year or so ago. They were really quick to make using Milliput modelling clay to make some carapace claws, ripper tentacles and some kind of flesh venting. One more pic after the jump. 

These are quite basic models. They are just fill in models until the GW model is released. If you guys are interested in a more detailed tutorial guide on how to make these let me know in the comments and I'll see what I can do.