I'm not sure that there's such a word as 'undersign'. As a signature is normally placed at the bottom of a document, the normal word is to 'sign', simply. Of course there is a word 'undersigned', a noun, meaning the person or persons who have signed the document: 'We the undersigned do hereby declare...' (I.e. 'We whose signatures you will see below...'

If you are asking whether 'sign' includes 'stamp', I would say the answer is no.

I think the word you need is "sign", not undersign.

Please sign this document here

(non-native)

edit: escoville war schneller.

Thank you very much for your answers.

The verb "to undersign" is in LEO and is translated as "unterschreiben", "unterzeichnen".

OTOH, the noun "the undersigned" is often used.

OED hat zwar undersign in der gewünschten Bedeutung, aber die Beispiele sind sehr begrenzt:

"  trans. To sign one's name below (a writing).

1580  C. Hollyband Treasurie French Tong   Soubsigner, to vnder~signe.

1770  L. Scrafton Refl. Govt. Indostan (ed. 2) 80   These words were written in his own hand, at the beginning of the Treaty, and were undersigned by him."

I wouldn't recognise to undersign or to underwrite as a synonym for to sign.

To underwrite has to do with insurance, and means to guarantee against financial risk, see http://lexicon.ft.com/Term?term=underwrite