Yes, possible. But it's unlikely that we will have resources to do that anytime soon.
On the other hand, we'd be happy to apply a patch that implements this function, if someone would provide it to us.
There's many aleeady implemented udf functions, for example
https://github.com/megastep/mysql-udf?files=1
there's more functions here, very interesting:
http://mysql-udf.sourceforge.net/
Hi,
we use UDF function "MEDIAN" from https://github.com/megastep/mysql-udf in our projects and it works well!
I think, that it could be a built-in part of MariaDB 
When we'll be implementing it, it should probably be not MEDIAN(), but a more generic PERCENTILE().
This is duplicate of MDEV-12985
@Sergei: why would you want to name it "PERCENTILE". Everybody know the mathematic function name median, so everybody will search it by that name.
But I'm hoping for that function to be integrated too.
Done. Both PERCENTILE and MEDIAN. See MDEV-12985
Thank you Sergei!
It's like a Christmas present ))