Do all words belong to conventional truth (Sammuti Sacca)? "Sanketa vacanam saccam - loka sammuti karanam
  Paramattha vacanam saccam - dhammanam tathalakkhanam"


According to the above quote, all the words (cenventional truth) made by living beings using any language considered as truth because they are enacted by living beings.

We use words to mention ultimate truth. For example when we say "Nibbana", the real dhamma or actual Nibbana exists but the word "Nibbana" has been enacted. The actual dhamma and the word used to refer it are two different things. The word may not contain the actual characteristics inherent to Nibbana. I suppose Shakyamuni Tatagata Lord Buddha used many words to explain ultimate truth (Citta, Cetasika, Rupa, Nibbana) because of this reason.

So my question is: Does the word "Nibbana" (as well as the other words used to mention ultimate truth) belong to the conventional truth while only the actual dhamma belong to the ultimate truth?
All words are like sign posts. The meaning of it is an idea that is in the mind of the person who is speaking it. And the communication will take place if the person who hears it has assigned the same meaning to the word in his/her mind.

Therefore the shared meaning has to be previously agreed upon or communication will not be very effective. As a result it is extremely difficult to communicate an idea (ex: enlightenment/dhamma) when the receiver has no data points to relate to. They become merely words. 

So yes, the words don't have the properties of what they refer to. For example Alan Watts once said that "You can't get wet from the word 'water.'".