Sorry to revive a dead thread, but on a similar note, there doesn't seem to be a way to associate a line item to a task through the API. If I have a task called "Some Task" attached to an existing project and do:
<?xml version="1.0"?>
<request method="estimate.create">
<estimate>
<client_id>15</client_id>
<lines>
<line>
<name>Some Task</name>
<type>Time</type>
</line>
</lines>
</estimate>
</request>
And then view the estimate in freshbooks, there are now two "Some Task" entries in the drop down. Ideally, I'd like to be able to optionally specify a task id, as it makes it much simpler to correlate time tracking info later on to see which tasks are over/under.