Are there any circuits to get output as a sum of a set of current sources?
Well considering your question, here is a general outline for something that we could make work. The ideal current sources represent the input sources, (note that they are fairly small and the assumption is that yours are as well). 

The configuration made by Q1 and Q2 is a simple current amplifier, where the output current at the collector is approximately the betas of the NPN and PNP times the input current. 

The collector currents of the PNP transistors both flow into R4, so effectively we have created a current summer. 

This isn't intended to be incredibly thorough, there are a lot of things left for you to consider. If this in turn leads to a working setup, please update the answer to reflect your changes. 

You would also need to regulate the voltage at the output. 







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