I am contributing regularly to my 401(k) plan, but the amount does not come close to the limits set by the federal government. Can I get my contributions to the limit between now and the end of the year? In other words, what happens if I am like $10,000 short of the limit but only have $5,000 in pay still due through the remaining paychecks due me? Can I put money I've saved into my 401(k) and get credit for it as pre-tax dollars?

No - sorry. Remember that a 401(k) is a form of deffered salary plan, so the only way to contribute is by defferring pay from your paychecks between now and the end of the year. You can't take money from other soures and add them to your current year contributions (although you may be able to roll in funds from another retirement plan into, but that's a different topic). You should certainly look into maxing your contributions as best you can - most plans have a max contribution they allow you to put into the 401(k) as a percent of salary - your plan administrator should be able to tell you what the max is.