How does a Minhag become a Minhag? More specific is it possible to enact new Minhagim in todays generation?
There are 2 types of minhagim (at least):
Many "minhagim" are halacha, but varying communities will follow different rulings based on the Rav's psak. Many new minhagim of this nature start because a new Rabbi comes to town and changes the custom of which psak to follow. I have seen this happen in a number of places. Right or wrong, the custom becomes significant enough that people who come to town should not act differently- like the evolution of any halacha. (Of course the minhag may only apply until the next Rabbi comes to town.
The other type of minhag enhances other mitzvos. Many minhagim are in the process of development simply because they are brought in the Mishna Berurah. How many people do we see giving tzedaka during v'atta moshel bakol because the MB mentioned that the ARI did it? Or how about minhagim that came about through tziyonut?
In short, minhag is whatever people do and these can change at any time.
Look at the באר היטב here, regarding the Minhag to bang or clap when we hear the name of Haman during קריאת המגילה, which details many conditions a Minhag must have in order to be considered a valid Minhag.
One of them is that there must be a source in Chazal (even a Pesikta) to be somech on.
(There is also a ש"ך which בלי נדר I will try to find later אי"ה.)