"I am not orginally born in Scotland neither in Ireland.I am not anyhow related to any Clans nor to any scot/irishman. But does this mean that I cannot wear a kilt?"

Absolutely not; if you want to wear a kilt, wear a kilt. Besides clan, district and corporate tartans, there are several free setts which aren't associated with any of the above, and self colored kilts with no tartan at all.

"I am thinking of making one because we are basicly going to start to practice somekind of a live-roleplaying game and are planning to follow the traditional way of scottish in 14th century."

Then you won't need a kilt; they didn't appear until 200 years later. In the 14th century Scots wore a long shirt and cloak, essentially like the Irish léine and brat, usually with a belt.

"They wear no clothes except their dyed shirts and a sort of light woollen rug of several colours."
Jean de Beaugué, L'histoire de la guerre dEcosse, describing Highlanders who were present at the siege of Haddington by the French in 1549.