Hi Azgorath,
Your request has been heard and will be included in the next version.
In the meantime, you can already use the following to help you out:
To add .chd support to any emulator:
Open mGalaxy > Data > Definition > Emulators.xml
There, find the emulator you want to work with (here, for example RetroArch) then the specific system and the specific option (here 'Arcade' and 'mame2000_libretro.dll')
<Emulator name="RetroArch" hidden="true">
<Systems>
<System name="Arcade" option="mame2000_libretro.dll" script="false" cmd="-f -L cores\mame2000_libretro.dll "%path\%file%ext"" extensions=".7z,.zip"/>
Add ",.chd" at then end of the extensions value
<System name="Arcade" option="mame2000_libretro.dll" script="false" cmd="-f -L cores\mame2000_libretro.dll "%path\%file%ext"" extensions=".7z,.zip,.chd"/>
...and voila!
The same goes for adding a core (if you do know the exact name of the core .dll)
Duplicate a line then edit the 'option' and 'cmd' values to the new core name.
So, for instance, duplicate that line
<System name="Arcade" option="mame2000_libretro.dll" script="false" cmd="-f -L cores\mame2000_libretro.dll "%path\%file%ext"" extensions=".7z,.zip,.chd"/>
and edit it that way (pseudo code)
<System name="Arcade" option="the_new_core_name_libretro.dll" script="false" cmd="-f -L cores\the_new_core_name_libretro.dll "%path\%file%ext"" extensions=".7z,.zip,.chd"/>
Warning: be particularly careful and attentive when editing this file, it is a crucial file for the proper functioning of mGalaxy