20% is the minimum mandatory withholding on a distribution like a pension buyout paid to you that is eligible for rollover. You can request that the payer withhold more. If the forms for requesting the distribution do not allow for requesting more, you can submit Form W-4P to the payer to request additional federal withholding:
https://www.irs.gov/pub/irs-pdf/fw4p.pdf
If the distribution has already been made with only 20% withheld for federal taxes, you can make a federal estimated tax payment as suggested by VolvoGirl. Similar things can be done with state withholding or estimated tax payments.
