It isn't a question of making the part in Fabrication, that can be done.  It is more a question of whether the parts physically exists  - and if they do, can the project afford using the pipe and fittings you will need.

I recommend asking the project's  engineer of record for submittal information for the pipe and fittings you need.  You might find that the engineer will change the design in order to stay with industry standard pipe and fittings - even if 18" is  available,  availability should be confirmed as it might be a special order for some suppliers.