I have a 36' span, mistakenly cut an I-joist to fit to a stairway opening that I already had. So I was wondering if I can splice the 2 piece 36' I-joist and make one or do I need to buy an other?

Unfortunately not. Trusses and I joists are designed to be used in 1 piece. Once you cut it you broke the glue lam top and bottom. Maybe you can sell it to another person who needs a smaller one.