Copy the file from original directory while in TWRP, to your SDcard directory, then using a computer edit the file, save it, copy the new file over the one in the SDcard.
Then in TWRP copy the modified file to the correct location
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Sorry.. do you mean I have to replace the fstab.qcom file in the forceencrypt.zip with the fstab.qcom file from system/vendor/etc?