In the latest official markdown plugin, you can put most wikitext into your markdown text that fits on
a single line. Unfortunately, this does
not include links [[ ]] because that markup resembles markdown. But you can use the link widget
<$link to"mytiddler/>
If you want to put markdown into your wikitext, you can transclude the rendered contents of a markdown tiddler into your text:
{{mymarkdowntiddler}}
AFAIK, no one has made a markdown-to-wikitext or vice-versa converter. You could possibly use HTML as an intermediary and then convert to one or the other.
Markdown is the de facto markup language used in virtually all modern apps. There is no prospect that the rest of the world will suddenly embrace wikitext. It would be great if we could work seamlessly with markdown, but I guess we have to use whatever work-arounds we can get.