First I manually set up one Kea reservation to serve as an example. Then I saved my config, and in a text editor I copied all the old <static> blocks and used find and replace to update their xml tags to Kea values. I deleted <cid> tags, and server tags <dnsserver/>, <winserver/>, and <ntpserver/>.
Each Kea reservation has its own uuid, so for each reservation I incremented the example value by 1, figuring it didn't matter if they were consecutive as long as they were unique. I added the subnet tag to each with cut and paste, saved all the reservation blocks into the Kea section of the config, and loaded the config back into OPNSense.
Worked perfectly and saved me a lot of setup time.