I've been searching for this but haven't found anything:
aren't there external variable speed motors that can be mounted side by side with a lens in order to convert a manual zoom into a powered zoom? I don't know what they're called ~

any reference is appreciated!

There are a number of makes including Heden, Preston and Arri, but a zoom servo motor or zoom motor would cover what you're describing. These are now digital motors that are controlled by a separate control used by either the camera operator or the 1st camera assistant.

Preston Cinema Systems (http://www.prestoncinema.com/products.html)
HEDN Engineering (http://www.heden-engineering.com/products.htm)
ARRI Group: Controlled Lens Motor CLM-3 (http://www.arri.com/camera/professional_camera_accessories/wireless_remote_system/controlled_lens_motor_clm_3.html)

thanks for the info. are there also attachments that can help make the manual zoom smoother? I'm thinking like a geared crank that would take several revolutions to zoom the lens just a small bit?

You can get a fluid drive for zoom lenses. these are made by Chrosziel and work extremely well. Some of the old 1960s & 70s Angenieux zoom lenses had a zoom crank fitted, although I found these devices rather underwhelming to use on the couple of occasions I came across them.