Chad,
There's no one answer to that question - you can go pretty much anyway you want with this. There are many, many different views of how piracy operates in Traveller.
One question, first: how did the cargo get on-board if it won't fit through the airlock? There must be some sort of hatch big enough for it to be on the ship in the first place. Evacuate the atmosphere from the hold (most ships have multiple compartments separated by bulkheads), pop those cargo hatches, perhaps turn off the artificial gravity in the hold to facilitate moving the cargo, and move it across. If the pirates do this kind of thing a lot (in other words they're real pirates, not just a desparate merchant who's turned to piracy in a pinch) they'll be experienced at this sort of thing, and probably have whatever is needed to protect most simple cargos for the short transition. Or maybe not and they have to improvise...
Some other options:
- The pirates execute the crew - "Dead men tell no tales." Pretty harsh and not good for the game, so not recommended.
- The pirates incapacitate the crew somehow. A lot of ways to do this: physical restraints (tieing them up or locking them in a storage room or something), drugging them, manipulating the artifical gravity to pin them to the floor (might involve making some modifications to the ship to permit that), securing them in a area and then partially evacuating the atmosphere until they pass out - lots of possibilities.
- The pirates keep the crew under guard. This is a lot riskier for the pirates and ties down some of their personnel, but it offers the crew a better chance to escape.
- The pirates take a hostage (or two, or three). They threaten to start by killing the hostage in some manner almost certain to succeed (gun to the head of a bound vicitm, bomb vest like the one Sandra Bullock wore in Speed, etc.) if the crew undertakes any form of resistance, alerts the authorities, etc. This could be combined with one of the other methods. Likely hostages include children (hey, they're pirates, they're bad guys), a critical member of the crew like a pilot or engineer (depending on the size & type of ship & the crew make-up), or the captain. Evil twist: the hostage gets moved aboard the pirate's boat, or their main vessel, or stuck in a vacc suit and tethered to the hull or otherwise kept apart from the crew.
- The pirates try to recruit all or some of the crew. Again, this could be combined with another option. Prospective recruits get moved under guard to a separate location and kept secure until the operation is finished.
A few other considerations:
- If the pirates have been doing this for a while, they're going to have the obvious ploys the crew may try covered. They won't be stupid. The old "Help, he's sick, open the door!" trick is likely to meet with a sleep gas grenade thrown into the room. They're going to be ready for resistance and will act to bring the crew under tight control somehow very quickly.
- The pirates may also disable key systems on the vessel. They might crash the computers, lock out the bridge or engineering controls, remove or damage some key component, etc. They'll do this do discourage resistance or pursuit.
Random gamemastering advice:
- If one of the characters tries something truly dumb ("I charge the one holding the gauss rifle and attack him with my bare hands,") let the chips fall where they may. I generally don't try to deliberately kill off a character, but the opposition should present a real threat and if someone does something patently stupid they will face the consequences. The first time something like this happens I'll fudge the rolls and just wound them severely.
- Try to pick a flow of events that will be entertaining. Don't be afraid to allow events to take a different direction from what you had already planned, and don't feel compelled to force things to go a particular way (usually called "railroading"). Whether or not the pirates give the characters some kind of opening is up to you. Just because they aren't stupid doesn't mean they can't make a mistake, or that the characters won't be successful in pulling off some kind of subterfuge.
Hope this helps!
- John