Direct import is impossible. Madden NFL 08 has no NCAA Football integration, shared roster format, or legend import feature. The games use different engines, attribute scales, and data structures.
Madden 08 includes NFL Legends (not NCAA) in "Madden Legends" roster: Jim Brown, Barry Sanders, Jerry Rice, Lawrence Taylor, Deion Sanders, etc. Unlock via:
Best Result: Spend 20 mins in Create-a-Player with a ratings spreadsheet. You'll get a more accurate version than any roster patch.