hi,
its the first time i heard of it so i needed to google it

if you look at your flash drive does it look like a normal extracted iso? at the root folder -> boot, sources folder etc ? and at the sources folder is there an install.WIM or install.ESD ?
if it looks like a normal iso it should be possible to use it via winreducer. since there is no option to extract from USB you will need to copy the files from your USB drive to your hdd (i.e. to WinReducerEX100\WORK\EXTRACT\ExtractedByWinReducer)
after that you can just select the folder at winreducer.
if you got an install.esd you will need to convert it via the wim converter to an install.wim (you can find the converter also on the main page of winreducer)