TYPO3: I need to add typoscript lines to object with a custom extension - typo3

Question

I added an image selection to a content element in my main extension with a custom extension, this field should now be added in the dataProcessing that is performed in the main extension, but it should be added in the custom extension :
tt_content.t3bs_carousel < lib.contentElement
tt_content.t3bs_carousel {
templateName = Carousel
templateRootPaths {
9 = {$plugin.tx_t3bs_main.view.templateRootPath}Content/
8 = EXT:t3bs_main/Resources/Private/Templates/Content/
}
dataProcessing {
10 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
10 {
references.fieldName = assets
as = assets
}
// this 20 should be added by the custom extension
20 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
20 {
references.fieldName = tx_t3bs_portrait_image
as = portrait
}
90 >
}
}
if I safe this in the main extension it works, I cannot find a typoscript that makes me add it in my custom extension, I tried to insert the entire object (since my custom extension is loaded after the main), then also only the missing part that needs to be added:
tt_content.t3bs_carousel.dataProcessing {
20 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
20 {
references.fieldName = tx_t3bs_portrait_image
as = portrait
}
}