I have a signed PDF and want to export it to a PDF/A with Libreoffice draw. But when I do that, it removes the signature from the File.
Of course it will invalidate the signature. The exported PDF is something generated by LibreOffice from the data it imported from some random source, that might happen to had been signed; but the end result is by no means is the same that was on import. This is normal, expected result.
Is there a posibility to sign the file while exporting it?
File|Export As|Export as PDF...; see Digital Signatures tab.