Yup. Quite true. Whole set of decals needs added to the fork truck. Everyone needs to know hand signals, crane operator card, etc. Load charts must be on board for the attachment, and the attachment must have the approval of the fork truck manufacturer to be considered legal for use on that particular powered industrial truck.
If you're talking about the "truss setter" boom for a LULL, however, that's a whole other story.
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.