If you add a Support Carrion Lancer into a squad of reanimates and you are engaged with an enemy unit can you do a melee attack and use the Carrion lancers ability to target another unit in LOS that is unengegaed. Meaning if there is a unit closing in on you behind a unit you are engaged with can you assign a blight token to them? Suppose your unit is a 3x3 and you are currently engaged with a 2x2. So LOS could be established on the far tray in the front rank.

As long as the your unit has clear line of sight to it's target and the target is not engaged, the skill from the Support Lancer works. It's not a ranged attack so it can be used while engaged.