My friends and I like to play with this rule:
Any unit not involved in close combat may fire into an ongoing close combat. However, once they have chosen to fire into close combat, the player rolls a
D6. On a roll of 4+, their shots hit the enemy and firing is carried on as usual. On a roll of 1-3 their own units are the victims and hits are resolved against them instead. Note that once the decision has been made to fire, the unit MUST fire all possible weapons; i.e. the player can’t take back the order to fire once he finds out his own units will be shot. Blast weapons use three shots when firing into close combat; blast markers are not used.
Of course you could change the rate at which the enemy is hit, from 4+ to 5+ or 6 if you feel that's necessary to preserve the balance of shootiness vs choppiness, as Blacksails would say.