The CData Excel Add-In for Basecamp supports standard SQL joins like the following examples.

Inner Join

An inner join selects only rows from both tables that match the join condition:

SELECT Projects.Name, ToDos.AssigneeName FROM Projects, ToDos WHERE Projects.Id=ToDos.ProjectId

Left Join

A left join selects all rows in the FROM table and only matching rows in the JOIN table:

SELECT Projects.Name, ToDos.AssigneeName FROM Projects LEFT OUTER JOIN ToDos ON Projects.Id=ToDos.ProjectId