On the OP, it's not possible at this stage - you an auto-assign from the build queue to existing fleets, but you assign lines, there's no 'smart allocation'. Although 'smart allocation' would be particularly difficult given the long lead time of ships and the brutality of naval battles in the game at the moment.
Also, once war's broken out, there's usually not a 'reserve' per se - most ships are generally doing something useful, even if it's as a fleet-in-being/counter.
I think this is something that has been discussed here before, and would be a good idea for a naval revamp in the future. I've certainly seen the idea floated before (come on, that was good) and I think it'd be great to have ships auto-assigned to 'frontline' fleets as and when older ships are destroyed.
@Axe99
Having thought about this a bit more, what I think could work well is to have fleets as 'buckets of ships', where a proportion are under maintenance, a proportion in service and damaged ships repairing, with missions set 'offensively' and 'defensively' (and offensive missions 'triggered' after preparation, a bit like battleplans). The total ships available for combat would then only be those in service, which would be influenced by a bunch of things (their range, available ports and their quality, that kind of thing). Thoughts still a bit formative though, so I haven't posted a suggestion per se.