Not sure if there is a process to submit such additions, but if you go in the MagicDB folder and edit the Ixalan.xml file, you should be able to get these cards showing up in Magicassistant.
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I was loading my collection in Magicassistant and the following cards were missing in the Ixalan set:
Not sure if there is a process to submit such additions, but if you go in the MagicDB folder and edit the Ixalan.xml file, you should be able to get these cards showing up in Magicassistant.