Hmm... Off the top of my head, here's some rules for taking people hostage in the way you're saying. Let me know if there's anything I've overlooked, or if you think the rules need ammending in some way.
Overpowering Enemies:
To overpower an enemy in melee and take them hostage make your attacks as normal. However, instead of rolling to wound, both models roll a d6 and add their Strength characteristic. If your total is equal or lower than his victim's, you fail to overpower them and nothing happens (you may try again for every successful hit). If his total is higher, then the hostage is disarmed and held.
Models that are holding enemies receive a 3+ save against all shooting attacks, with any passes being transferred to the held model (you don't get a save against blast and template hits, instead they simply hit both models). In addition, enemies must usually pass a leadership test at -2 to fire at a model holding an ally (there are some exceptions to this; no Tau will shoot an Ethereal, Commissars auto-pass, etc.). The Shot-Of-A-Lifetime heroic action ignores the save, but not the leadership test (if the test is failed, the Narrative Points are not spent)
If a model with a hostage is assaulted, they immediately drop the hostage. The hostage may not attack in that turn and is hit automatically, but otherwise behaves as normal from that point on. If the hostage-taker is killed by shooting, the hostage is dropped, but may not make any more actions that turn.
Roleplaying Actions
Non-Combat actions like demanding a surrender will be determined on a case-by-case basis. Sometimes I'll roll dice to determine the outcome, sometimes I'll just decide how it goes. I won't tell you your chances in advance, but these actions won't usually cost you anything either (Talking Is A Free Action, after all).
