I have done a short search of this group and found at least one post stating
that it is ok to splice the wires inside the panel. The question is: what
is the standard way to do this? Can I simply use bare, crimped butt splice
connectors on the ground wires and insulated ones on the neutrals? Is there

I would not like having too much splices in a main panel, wheter legal
or not, as all the Marr connectors will be in your way when doing
future work (especially if you need to splice 10g or 8g wires). And
even the connectors are known as safe, I wouldn't want to see one
falling off in the breaker box over time.

So I liked Speedy Jim's suggestion of using the old panel a lot, if
that is possible.

I'm no electrician, so take this as a potential idea more than as an
advice: A solution if you're really out of space might be to install a
lot of small covered rectangular metal boxes in your work area (those
used for outlets) and use them to make your splices. I think you can
legally cram the wires from two circuits in the deeper ones, but check
the specs of the boxes first to be sure you can. You then just have to
bring a clean wire inside the new breaker box. It will not be nice
when looking outside the breaker box, but it will make any future work
in it easier.

Max.