This question has been answered by BobRay. See the first response.
<?php
$users = $modx->getCollection('modUser');
foreach ($users as $user) {
$profile = $user->getOne('Profile');
$extendedFields = $profile->get('extended;)
$extendedFields['newField'] = 'NewValue'; // could be ""
$profile->set('extended', $extendedFields);
$profile->save();
}
<input type="hidden" name="januarySpaces" value="" />
<input type="hidden" name="januaryTables" value="" />
<input type="hidden" name="januaryChairs" value="" />
<input type="hidden" name="januaryNeedsElectric" value="" />
<input type="hidden" name="januaryMisc" value="" />