Marine was carried on stretcher improvised from a machine gun. 这句英语的定语前加个which可以吗？

如果要加which，必须还要再加一个was，因为加了which就是一个句子，定语从句。句子必须保持主、谓、宾（或表语）完整，不能缺成分：

Marine was carried on stretcher which was improvised from a machine gun.

目前例句不加which，这是短语，不是句子，所以可以省略一些成分，但作用和和which was定语从句一样起修饰作用。