In the example given by the OP this is how I would handle it…

Each mook is attempting to run past the defender. They take a Run Move Action, moving as far as they choose up to their Run Move distance; as each mook moves into a square adjacent to the defender he is momentarily engaged (by default), and the defender receives a +20 bonus to WS to strike a running target. If successfully struck each mook may use a Reaction to Dodge/Parry the hit. Any damage is then resolved and if the mook survives he finishes his move into the next chamber and is not engaged. Rinse and repeat with each successive mook. If the defender wants to prevent anyone from running through the door he might use unarmed combat rather than lethal force and attempt to Grapple the first mook in an attempt to hold him fast and effectively plug the door with struggling bodies.

Striking a foe that "retreats" from and engagement is a Free Standard Attack. As a GM I would allow only two Free Standard Attacks, as the GM is suggested to use his discretion regarding the number of Free Actions PCs may perform in a single Round my ruling would be that 2 Free Standard Attacks equal one full Action (as each Standard Attack is normally a Half Action, and striking a fleeing/running enemy is more taxing than simply dropping an item or speaking a few words).

So, in the OP example, only the first two mooks attempting to bypass the defender have anything to worry about (insofar as lethal force).