There is no general approach so far, I didn't had time to complete my MDXfind to Hashes.org conversion script.
After you run your founds through mdsplit you have one file per algorithm which is good, as you have to upload separately anyway as only one algorithm per upload is accepted on hashes.org.
So for every found file you need to convert the algorithm into hashes.org format. There are even some algorithms from MDXfind where this is not easily possible. There is no general way, but in the most cases it's pretty similar, if you haven't take a look at the algorithms page so far, I recommend to read through it, to understand how the hashes.org algorithm works: https://hashes.org/algorithms.php
If you're not sure about the conversion for an algorithm, feel free to ask here.