I am having a similar issue, in that, I want to create a VM that accesses the internal extFS4 partition (also disk0s4), but it is not listed in the drop down menu, as shown in a similar screen grab:
This used to work in Parallels 10, but with older versions of OS X, so I assume the problem has arisen with Apple's implementation of APFS and SIP.
I've spoken with Parallels support, even letting them take control of my computer, but they said "This is not possible," and "We will look into it further, since you said you were able to do it in previous versions of Parallels." Guess we'll wait and see.
I have seen other posts about more-or-less tricking Parallels by changing the GPT Typecode, as seen here, but I have not tried this yet, as I don't know where to make this change. Hope this helps move us closer to a solution.
Last edited: Mar 18, 2018