PLY has a somewhat complex system of defining tokens, lexems, grammar etc. but I would like to create a parse tree using an already existing Ruby's file - parse.y.
Is there a way to read the file parse.y and create a parse tree for Ruby program in PLY?
Short answer: no.
That file contains 13,479 lines; the actual grammar 769 lines, including 46 Mid-Rule Actions (MRAs), so there are close to 13 thousand lines of C code which would have to be rewritten in Python in order to reproduce the functionality. That functionality includes the lexical analyser, which is about a thousand lines of C code plus supporting functions. (If you think Ply's method of defining lexical analysis is complicated, wait until you try to reproduce a hand-written analyser written in C. :-) )
I extracted the grammar from that file using bison (although I had to edit the file a bit in order for bison to not choke on it; I don't know where the Makefile is in that source repository but I presume that it includes a preprocessing step to make a valid bison grammar file out of parse.y). So you could do that, too, and use the result as the basis of a Ply grammar. You might be able to automate the construction of the grammar, but my guess is that you would still have to do quite a lot of work by hand, and if you don't have at least some experience in writing parsers that work is not going to simple. (It may be educational, though.)
Good luck with your project.