Replacement or parallel operation of Diesel Generator(existing) + solar PV(proposed) + grid supply. Is it possible?

Yes this is quite possible and common, but it depends on how big of a generator you're trying to replace. Currently available battery systems have practical limits that make them most effective for smaller, distributed systems rather than centralized power plants.

We can provide equipment and integrated systems for small household up to village micro-grid scale (up to about 20kW peak power) PV/diesel/grid hybrid systems.

Eric
Solar Nexus International

Indeed generators are commonly possible the details is in the particular DC/ AC inverters model being used which has the features that functions with duel processing capabilities systems designs called Hybrid electrical generation that operate between having the diesel ,gas, or LP electrical generators connect to an battery bank which in addition to being connected with both / or either one type of method that combines solar panels and /or wind generators for the sole purposes as Eric had mentioned for recharging the battery banks the benefits and advantages are found to reduce fuel consumption need for operation of such diesel generators and of it fossil fuel engines types completely .

Note : hydrogen fuel cells are on the market which offers 7 times the discharging rate of the conventional deep cycle batteries.