Use the CUSIP converter program--cusiper. It can prefix zeros to CUSIPs whose preceding zeros were truncated in MS Excel, and then.
1) Convert 6-digit CUSIPs to 8-digit CUSIPs for CRSP, I/B/E/S and Execucomp.
2) OR keep it as 9-digit CUSIPs for Compustat.
3) OR from 8-digit CUSIPs from, say, Thomson, to CUSIPs with leading zeros and replaced zeros for last digit.
4) OR from 8-digit CUSIPs to 6-digit CUSIPs

Or if you prefer Excel, you can go to Converting Company Level CUSIPs to CRSP CUSIPs for instructions on how to convert 6-digit CUSIPs to 8-digit CUSIPs.

For more information about CUSIP, go to CUSIP Service Bureau and select "What's CUSIP?".