Here are the commands I could find. I don't think any of them do that.
Usage: SubtitleEdit /convert <pattern> <name-of-format-without-spaces> [<optional-parameters>]
pattern:
one or more file name patterns separated by commas
relative patterns are relative to /inputfolder if specified
optional-parameters:
/offset:hh:mm:ss:ms
/fps:<frame rate>
/targetfps:<frame rate>
/encoding:<encoding name>
/pac-codepage:<code page>
/track-number:<comma separated track number list>
/resolution:<width>x<height>
/inputfolder:<folder name>
/outputfolder:<folder name>
/overwrite
/forcedonly
/teletextonly
/multiplereplace:<comma separated file name list> ('.' represents the default replace rules)
/multiplereplace (equivalent to /multiplereplace:.)
/ebuheaderfile:<file name>
/ocrengine:<ocr engine> ("tesseract"/"nOCR")
/renumber:<starting number>
/adjustduration:<ms>
The following operations are applied in command line order
from left to right, and can be specified multiple times.
/ApplyDurationLimits
/FixCommonErrors
/RemoveLineBreaks
/MergeSameTimeCodes
/MergeSameTexts
/MergeShortLines
/FixRtlViaUnicodeChars
/RemoveUnicodeControlChars
/ReverseRtlStartEnd
/RemoveFormatting
/RemoveTextForHI
/RedoCasing
/BalanceLines
Example: SubtitleEdit /convert *.srt sami
Show this usage message: SubtitleEdit /help
List available formats: SubtitleEdit /help formats
Regardless, I've since solved the problem by creating a second but private share that points to the same location. So I can still have the read only one for guest access, but the other requires credentials. Didn't know I could do that before.