WFRP 2nd Edition Core Rulebook, p130:
Shooting into Melee
Sometimes, a target will already be engaged in melee combat. This
makes the target more difficult to hit. A character can make a ranged
attack at an opponent engaged in melee, but suffers a –20% Ballistic
I believe this rule is directly applicable in 1st edition as well.
YMMV in 3rd Edition as the combat mechanic changed significantly between editions. The dice pool mechanic would require some "translation" by the WFRP3e GM to utilize the -20% modifier from previous editions. However, if the WFRP3e RAW do not call out a modifier, previous editions provide a baseline for a "house rule."
Research suggests that during combat resolution, the GM will assign challenge and misfortune dice as follows:
Next the GM assigns any relevant challenge or misfortune dice to the pool. The default difficulty for Melee Attack and Ranged Attack actions is Easy (1d), but may be modified by a variety of factors. Based on the situation, the GM may decide the action in question is better served as an unopposed or opposed check.
This step is where the target’s Defence value is added to the dice pool. One misfortune die is added for every point of the target’s Defence. A target’s Defence value is generally determined by its armour or innate abilities to avoid or shrug off attacks.
After seeing how many challenge dice or misfortune dice the GM adds to the dice pool, the player has one more opportunity to spend fortune points to add fortune dice to the pool. Each fortune point spent adds one fortune die to the pool. Spent fortune points are returned to the general supply.
WFRP2e describes difficulty modifiers to combat actions in a similar vein. If Easy (1d), maps 1-to-1 with the WFRP2e difficulty chart, a -20% would balance to an Average difficulty.
The precise "houseruling" is left as an exercise for the WFRP3e GM.