When a block is found by a pool, transaction payments from verification of online transactions (0.00001 BTC) are added to the block so that the block is more than 25 BTC. Who does this addition? the blockchain or the pool operator.

How will the blockchain know how much to add. If it is the pool operator then it means some "revenue", however small, is being generated all the time.

Is it possible to mine just for the fees?