No, Lewisham extension is described as later phase.
The line doesn’t even have a fourth rail in large parts, I doubt the signalling has tripcocks. What benefit would you get from spending such a huge amount of money to make this happen?
Better frequency of trains?
The Underground isn’t some golden token though. More frequent and longer trains could be provided without huge infrastructure cost by just ordering more overground trains. That that hasn’t happened says all that needs to be.
The Bakerloo (or more strictly Baker Street and Waterloo Railway) originally only had platforms that were around 290 feet (88 metres) the tunnel platforms were subsequently lengthened to nearer 370 feet (113 metres). On most of the platforms, generally can see where the tunnel rings of the platform extensions start as they were built around original tube size rings, so there is kink in the platform walls/ceilings
There are many places it should fit and doesn't. You're of course ignoring that there isn't fourth rail or tripcocks, and that it isn't all about clearance for a regular service, DOO monitors, platform gaps, the list goes on. If you wanted to spend the money why wouldn't you just order more 710s? What benefit does a costly extension bring that that doesnt? But this is probably getting quite speculative.
This is going to be a headache of sorts should the Bakerloo Line pursue level boarding for the deep level platforms. The line might need to raise all its platforms on the underground section in order to be of the same/similar height to the platforms shared with Overground. At least with the extension, if built, they could be built to the new height.