IMatch automatically maps from XMP hierarchical subject to flat XMP keywords and legacy IPTC keywords during write-back, and, for convenience, automatically in the database every time you update keywords. Under Edit > Preferences > Metadata you can control how IMatch
flattens hierarchical keywords during these steps.
Usually you have no levels in flat keywords. And if you configure IMatch to write hierarchies into flat keywords, it will always use the standard | pipe separator. You can only select arbitrary keyword separators for import.
If your "server" does not allow you to dial in the standard | separator, the only way I see is to manually replace the | in the flat keywords with \ using the
Metadata Mechanic app. I would recommend to do this on
copies of the images you have created before (in a separate folder).
Manually changing flat keywords may confuse the keyword mapping in IMatch (which always works from hierarchical keywords to flat keywords). Your changes may also be wiped out during the next write-back or re-import. Working against the automatic keyword management may be risky, which is why I would a) do it on a copy of the images and b) then delete these copies after uploading.