[Typo3] How to extend tbl_cms for loadTCA('fe_users')

[Christian Stuedemann]

Dear all,

I've made a BE-extension 'csextfeadr' that extends the fe_users table by
some additional columns. That works fine. Now I'm using
t3lib_div::loadTCA('fe_users') to read the column's configuration in a
frontend extension. I get all configs of the standard columns noted in
typo3/sysext/cms/tbl_cms.php, but I don't get the additional columns,
because they're not part of the tbl_cms.php file.
How can I extend the content of tbl_cms.php without touching the
original file so that loadTCA('fe_users') gets all the data?!

Thanks a lot and best regards,
Christian