可以的  就直接给主编或编辑写邮件  如实说明情况 问问能不能多给2周或1个月之类的
语气 恳切一些  就好了

一般编辑在给你的邮件中会给出修改稿提交的一个截止日期，还会特意说明，如果您不能在规定时间内完成，可以和编辑说明，延长修改时间。
您的论文需要大修，要补实验，耗用时间较长，可以跟编辑联系，说明实际情况，请求延长提交时间，编辑一般都会同意的。
祝好！

为了提高论文的学术价值，可以写信给编辑申请延期返稿。楼主可以参考以下模板：

Dear ***,

    Thank you very much for your email of *, 2016, with regard to my manuscript (Ms. Ref. No.: ***) together with the comments from the reviewers. During these time, we are revising the relevant part in the original manuscript according the suggestions of the reviewers. Unfortunately, we fonud that this work can not be completed within 20 days. Therefore, may we request to resubmit a revised manuscript before *, 2016? If that is possible, we are so  appreciate your kindness!

    Yours Sincerely,
    ***

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