We have a distribution Transformer of 2.8 X 1.5m, we will replace it with a new transformer whose base will need to be 3.3 x3 m. we intend to use the existing pad. is it ok to just put some dowel in it and just extend the existing pad?
If you have to do thatm I would consider encapsulating the old with the new, also using dowels as you suggest as well as a lot of other reinforcing. You may have a problem developing the bottom bars, depending on the loads you have.