combining phototransistors
Sounds like all you need is basic current summing.  This can be as easy as connecting them all to the same resistor.

The current thru a phototransistor is largely independent of the voltage accross it, so the voltage on the single resistor won't effect the individual currents.  To be clear, connect all the photoresistors in parallel between a power supply and this resistor.  The other end of the resistor goes to ground.  The voltage accross the resistor will be proportional to the sum of all the phototransistor currents.

Chose the resistor based on the maximum brightness all the phototransistors would experience that you care to measure.  Let's say each photoresistor allows 1mA to flow under whatever your maximum light conditions are, and that you have 8 such phototransistors connected in parallel.  That means the maximum current thru the resistor would be 8mA.  Let's say you want that to result in a 0 to 3.3V signal, and the phototransistors are connected to a 10V supply.  3.3V / 8mA = 413 Ohms, so chose the next common size down so that you still have a little headroom at max brightness.

Look at the datasheet of the phototransistors carefully to see what the actual current will be, not just the 1mA I picked out of the air.  Photodiodes usually only allow uA to flow, but with a phototransistor there is gain and only the datasheet can tell you what that is.  It's also a good idea to do some experiments with your intended light conditions since humans are rather bad at guessing light levels.  Make sure that with maximum light there is still enough voltage accross the transistors for them to operate in roughly the constant current region.