Okay if i understand you correctly, you want to create an item accessible to multiple companies? If that’s the case then this is already standard in ERPNext as the items list is global which means every company in ERPNext can access all items.
If it is to limit the item to specific company then i cannot help you there as i don’t know how, but if it’s for accounting/journal entry purposes then under the sales, purchases and accounting details, add rows and set the companies and ledgers accordingly. So when the items are used, it will automatically filter the company’s accounting entries based on the company set in the doctype.
I hope this helps.