Extending joists to reach a new beam. What code to review?
Use beam sizing software. There are ones available for free.

All things can designed outside of the "code" which is what the "code" defaults too if it is not prescribed. We usually just have plans and beam calcs to match that all get approved by our building officials and are not "in" the code.

I am not an engineer and it is allowed in my jurisdiction. Other jurisdictions the beams must be calc'd by an engineer. Point is, you do not have to be confined to the prescribed "code" per se. You can "design" it