Your question already assumes that the cargo ship is weaponless, and we'll also assume for the sake of simplicity that the pirates have already taken care of any escorts that might be around, so they aren't in any danger of getting blown out of the figurative water. The problem now is - they need to get the precious cargo off the cargo ship. For that, I see two possible ways to enter the ship:
Boarding Through Airlocks
The most obvious way is to simply slip spacesuits on and pop open an airlock from the outside. Any space-faring vessel should have some type of de-pressurizing zone the pirates can use to their advantage to access and enter the ship.
Depending on your priates' style, this can be done either after they've done precision damage to render the ship immobile, or mid-flight and stealthily to avoid any detection whatsoever.
Make-Your-Own-Airlock
If for some reason the ship doesn't have an entryway, they could make one on their own. Simply give your pirates an attachment to the ship that manually digs into the hull, preferably 'locking-on' like a claw, that allows the pirates to connect one ship to another for easy boarding.
It doesn't matter if they then leave a huge hole in the ship's hull after they leave - they're pirates. They got the cargo. Let the poor saps with a hole in their ship worry about that.