Time entered through CSV Importer always overwrites, never accumulates

I'm guessing that there's a way to accumulate time on a JIRA item, but I'm not getting it. I started with the example CSV given in the help pages:

Summary,Worklog
Only time spent (one hour),3600
With a date and an author,2012-02-10 12:30:10;wseliga;120
With an additional comment,Testing took me 3 days;2012-02-10 12:30:10;wseliga;259200

And I adopted it to work as three updates to my own sample project as follows:

Issue Key,Project Name,Project Key,Summary,Worklog
TEST-34,Test project,TEST,,3600
TEST-34,Test project,TEST,,20130210123010;mbrown;120
TEST-34,Test project,TEST,,Testing took me 3 days;20130211123010;mbrown;259200

The intent here is to have all three lines update a single JIRA item, TEST-34, in my project named "Test project". I had to adjust the date/time strings to match my desired format, but that was minor. Everything worked correctly except that the overall Time Spent on TEST-34 was 72 Hours, rather than 73 Hours 2 Minutes as expected. (3600 + 120 + 259200 should add to 73 hours 2 minutes, but instead the last entry of 295200 just forced the total time to 72 hours.)

Here is the CSV mapping file I used. You'll note that the worklog column is mapped to field.worklog, not to Time Spent. Out of curiosity, how many fields can go into the Worklog column separated by semi-colons?

{ "field.Project Name" : "project.name", "date.import.format" : "yyyyMMddHHmmss", "field.Worklog" : "worklog", "field.Issue Key" : "issuekey", "field.Project Key" : "project.key", "user.email.suffix" : "@medaptus.com", "mapfromcsv" : "true", "field.Summary" : "summary" }

Copied from https://ecosystem.atlassian.net/browse/JIM-908