Can you add on to existing underground duct banks? 1

Don't know of a reason not to extend the duct. You would have to chip the concrete back enough to get to the duct. Problem is not to damage the existing cable.

How about just building a new duct bank right beside the old one?

MAKE SURE that you document/update all of your exisitng records.

You cannot splice conductors in a conduit, so I assume you would pull out the existing conductors, intercept and extend the duct bank to the new location and re-pull all new conductors.

If this is your intent, it is possible. You would need to specify the chipping back of the concrete enough to expose the conduit beyond where, ( it is assumed), the elbows currently bring it up vertically through the floor to the existing motors, and then extend the horizontal conduit runs to your new equipment locations.

I don't know your configuration, but I have also left the risers in place, installed an empty switchgear section to use as a pullbox above grade and extended them either back down underground, or overhead. Not the most elegant solution, but effective and one of the least costly options as you would utilize most of the existing duct bank.