[TYPO3-seminars] Adding localized strings

[rsbrux]

First off, many thanks for the excellent documentation for this plugin!  I
am using the latest TER version under TYPO3 4.5.40

Unfortunately, the usual approach for customizing localization strings
(well documented at
http://docs.typo3.org/typo3cms/extensions/seminars/ExtSeminarManager/Configu
ration/ChangingTheLocalizedStrings/Index.html)

is not working as I expected.  For example, I would like to implement
separate field labels for the fields:

tx_seminars_pi1_registration_editor_first_name

and

tx_seminars_pi1_registration_editor_last_name

in the registration form.

This works if I add the corresponding placeholders to my customized
RegistrationEditor.html (under fileadmin/templates, as recommended in
http://docs.typo3.org/typo3cms/extensions/seminars/ExtSeminarManager/Configu
ration/ChangingTheHtmlTemplate/Index.html) and define the new labels in
pi1/locallang.xml, but not if I define them in TS setup.

I can replace the values of strings that are already declared in
pi1/locallang.xml by adding them to TS setup, but any values not already
declared in pi1/locallang.xml are not substituted.

What is the proper way to add labels without modifying the extension's own
locallang.xml files?