Again, I'm new here so I don't know whether this is the right place to ask this (most likely amateur) question.
Don't worry, this is the right place to ask
I'm trying to mod Morrowind specifically, and a couple major mods require DirectX 9.
Can I install direct X via porting key?
or would I have to get crossover and go that route?
Most probably, the actual Wine implementation of DirectX in the port should be enough to make Morrowind run, so firstly I recommend you to try the mods directly, without changing anything in the port.
In case that don't work, here is how you do it: right-click the game in PK and go to Wineskin Tools -> Launch Wineskin. A new window will appear. Now go to Advanced -> Tools -> Install Winetrick. In the other window that will appear you will be able to install winetricks, which are the equivalents to Windows's dependencies like DirectX. Here there are three possibilities of winetricks that should work for you (in case one of them don't work, you will have to remove the port and create it again to try the new one):
In case none of them works, let we know which is the mod. Someone else might have used it in the Morrowind port already and may know how to install it.