Can I splice a main neutral in a new feed thru (meter panel) box?
The code allows you to splice service entrance conductors (230.46) 2011 code book.  But you cannot splice them in the meter can.  You have to use a weatherproof box seperate from the meter can; if installed outside, and weatherproof fittings.  The splice must be made with approved equipment.  Remember a chain is as strong as it's weakest link.  If you ever have a problem with the feed it will be thewre.  Best thing to do is re-run the line if it's economical it would be best.