Check C:\Program Files\Repetier-Host\plugins\RRFirmwares there is a file marlineeprom.xml
I think you need
<entry type="float">
<description unit="[mm]">Home offset X</description>
<set>M206 X@</set>
<detect>M206.X(-?\d.?\d)</detect>
</entry>
<entry type="float">
<description unit="[mm]">Home offset Y</description>
<set>M206 Y@</set>
<detect>M206.*Y(-?\d.?\d)</detect>
</entry>
<entry type="float">
<description unit="[mm]">Home offset Z</description>
<set>M206 Z@</set>
<detect>M206.Z(-?\d.?\d*)</detect>
</entry>
originally there are min="0" preventing the negative values. Have updated it for next release.