[{"offers":[{"name":"min","prices":[8,15,18]},{"name":"max","prices":[9,11,18]}],"nl":[2,4,6,7]}]

db.collection.aggregate([{$set:{offers:{$cond:[{$in:[7,"$nl"]},{$concatArrays:["$offers",[]]},{$concatArrays:["$offers",[{"name":"minimal","prices":[]}]]}]}}}])