I have an exiting 8 ft opening in a load bearing wall. I want to extend it by 1.5 feet on one end. Can I just build a new small section on the one end, or do I need to replace the existing beam with a new 9.5 foot beam.

Can you explain 'build a new small section on the one end?'

The beam has to not only go all the way across and have adequate post support under it at each end, it also has to be the right depth for the load it's carrying, which depends on what's above it. Hopefully it will be the same size lumber, just longer.