How do I add an additional database for migration?
You are on the right track. You can use

services:
  legacydatabase:
    type: mysql


And do a lando rebuild to spin up a second DB server. If you did not specify the credentials you can check them with lando info. 

To use db-import (or export) you have to specify which host to use. By default it uses 'database' and not legacydatabase or whatever you called your second DB server. As per the documentation:
Options:
  --host, -h      The database service to use                  [default: "database"]

So the command to use in your case would be:

lando db-import --host legacydatabase [your_mysql_file]