How do I add an additional database for migration? I have a Drupal 8 site running in Lando. I need to have the 8 site connect to a Drupal 7 database so I can run a migration. 

My understanding is I can import an additional database through Lando, but from the documentation I cannot figure out exactly how to do this. 

I've tried adding this to my .lando.yml:

services:
  legacydatabase:
    type: mysql
    creds:
      user: root
      password:
      database: legacy


and then multiple attempts at the lando db-import command to no avail.

I need to know what exactly goes in .lando.yml and then what exactly goes in the lando db-import arguments to get this working.
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]