If you have another OSR game that has a druid class, you could port that into the game; just tweek the spells and class abilities so they are comparable to the existing LotFP classes. Alternately, you could use the LotFP cleric as the basis of a druid class. Go through the cleric spell list and eliminate any spell dealing with good or evil/law or chaos (Detect Evil, Protection from Evil, Turn Undead, for example). You could then adapt a few M-U spells to replace the eliminated cleric spells (Wall of Fog, Dark Vision, Gust of Wind, Growth of Plants, Hallucinatory Terrain, and Control Weather among others could be appropriate for a druid. The main consideration would to have no more than 10 spells per level and that the spells are not more powerful than a cleric spell of the same level.
Regards,
V