Susan:
I just exported 15,000 items to a CSV file (2 columns = Item #, Pick Seq), edited it to change some Picking Sequences and successfully imported it back in.

What version do you have installed? There's a Hot Fix dated Dec 24, 2010. It refers to solving an import problem with record-locking. Your Adagio reseller would be able to deliver that to you.

The message about the new item in the error log is normal. It just indicates that the process found a new item but skipped it since you have chosen to not import new items.