Unfortunately it isn't terribly simple to do, but it is possible. Here's a whitepaper explaining the math behind it: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070017872_2007014421.pdf
Check out the Unity3D Wiki page (The below code sample is from the same article): http://wiki.unity3d.com/index.php/Averaging_Quaternions_and_Vectors
//Get an average (mean) from more then two quaternions (with two, slerp would be used).//Note: this only works if all the quaternions are relatively close together.//Usage: //-Cumulative is an external Vector4 which holds all the added x y z and w components.//-newRotation is the next rotation to be added to the average pool//-firstRotation is the first quaternion of the array to be averaged//-addAmount holds the total amount of quaternions which are currently added//This function returns the current average quaternionpublic static Quaternion AverageQuaternion(ref Vector4 cumulative, Quaternion newRotation, Quaternion firstRotation, int addAmount){ float w = 0.0f; float x = 0.0f; float y = 0.0f; float z = 0.0f; //Before we add the new rotation to the average (mean), we have to check whether the quaternion has to be inverted. Because //q and -q are the same rotation, but cannot be averaged, we have to make sure they are all the same. if(!Math3d.AreQuaternionsClose(newRotation, firstRotation)){ newRotation = Math3d.InverseSignQuaternion(newRotation); } //Average the values float addDet = 1f/(float)addAmount; cumulative.w += newRotation.w; w = cumulative.w * addDet; cumulative.x += newRotation.x; x = cumulative.x * addDet; cumulative.y += newRotation.y; y = cumulative.y * addDet; cumulative.z += newRotation.z; z = cumulative.z * addDet; //note: if speed is an issue, you can skip the normalization step return NormalizeQuaternion(x, y, z, w);}public static Quaternion NormalizeQuaternion(float x, float y, float z, float w){ float lengthD = 1.0f / (w*w + x*x + y*y + z*z); w *= lengthD; x *= lengthD; y *= lengthD; z *= lengthD; return new Quaternion(x, y, z, w);}//Changes the sign of the quaternion components. This is not the same as the inverse.public static Quaternion InverseSignQuaternion(Quaternion q){ return new Quaternion(-q.x, -q.y, -q.z, -q.w);}//Returns true if the two input quaternions are close to each other. This can//be used to check whether or not one of two quaternions which are supposed to//be very similar but has its component signs reversed (q has the same rotation as//-q)public static bool AreQuaternionsClose(Quaternion q1, Quaternion q2){ float dot = Quaternion.Dot(q1, q2); if(dot < 0.0f){ return false; } else{ return true; }}Also this post: http://forum.unity3d.com/threads/86898-Average-quaternions