hi.i'm new to the site and i'd like to ask something. is it legal to have an employee sign a mandatory arbitration agreement before getting hired?

If the position is a union based position, then signing the arbitration agreement is a almost always a prerequisite before joining the union.

If the position is not union based, then its a matter of contract law. Under contract law, as long as all the elements of a contract exist and the spirit of the agreement is legal, just about anything can be negotiated. So, an employer can require an applicant to agree to arbitration prior to employment.

Employers often require applicants to submit to urine testing or credit and criminal checks prior to hire. These are all conditions to employment.

Even if an arbitration agreement is in place, the federal government can always set aside the arbitration agreement and litigate issues on behalf of the employees if they deem necessary. See attached link for more details.
http://www.dol.gov/sol/media/memos/August9.htm