Historically, the various sign languages (SL) have not developed an own writing system; nevertheless, some systems exist, among which the SignWriting (SW) is a powerful and flexible one. In this paper, we present the mechanisms adopted by signers of the Italian Sign Language (LIS), expert users of SW, to modify the standard SW glyphs and increase their writing skills and/or represent peculiar linguistic phenomena. We identify these glyphs and show which characteristics make them "acceptable" by the expert community. Eventually, we analyze the potentialities of these glyphs in hand writing and in computer-assisted writing, focusing on SWift, a software designed to allow the electronic writing-down of user-modified glyphs.