Like the previous user mentioned, aslong as you have the requirede skills you are allowed to board it. Many pirating corporations who have trouble in financially funding their own manufacturing of combat ships use this method as compensation. As a rep of a PMC, i would suggest you not to reason being is that there are alot of goodie-two-shoe corporations out there to patrol like cray. You steal something and out of the blue comes flying a group of mobilised frigates and cruisers.
Yet again, like mentioned above, co-operation and hijack through force is another method, but it's more time consuming. Like said, i wouldn't recommend this if you have the financial backing.