I want to start building up a stronger population for a bigger cluster this winter.
Ok - maybe that's what
you want - but is it what the bees have in mind right now ? Bees are a bit like cats - unlike dogs and horses you can't impose your will on them - they'll do what
they want to do and in their own time. The 'trick' to managing bees is inducement - to give them a reason to behave as you wish, but in their best interests of course.
Simply inserting another comb into the brood nest may or may not achieve more brood - indeed, they may even resent an interference to the otherwise stable comb situation which they've created.
If I was faced with this situation, then I'd either a) leave well alone, or b) insert a fully-drawn comb at the side of the brood nest - that way, the bees will themselves decide to either enlarge the brood nest, or move stores into that comb from elsewhere. But - it will be
their choice.
I'd be careful with feed at this stage - fairly small amounts every few days is what I'd give (if anything), as a constant supply of light syrup may fool them into thinking that a second flow has started, with unpredictable results.
LJ