Can i put the submitted paper in the arxiv? I'm not so familiar about the regulations regarding arxiv and conference papers.

Recently i submitted a conference paper and it's under review, but i couldn't put all the supplementary materials (proofs and extra analysis, pseudo-code etc) in the submitted paper due to the limited space!

So legally can i put the complete version in arxive, hoping if the reviewer search for the related works he might notice the complete version by chance. Then he can benefit from the extra supplementary materials provided there to better understand the submitted version?

Also arxiv is like putting the paper there, or you also need to wait for the acceptance from arxiv?
If you have just submitted the paper, chances are you haven't given the rights over to the committee. As long as you haven't agreed to some terms and conditions that gives temporary ownership to the conference, then you should still have ownership of the paper and can do what you want with it. 

If you want to be doubly sure, a short email to the conference proceedings editor can erase any doubt you may have.