Class/Sub-Class Creation Synergy

FGU/5E

Let's say you have the core/optional rule books (PHB, Tasha's and Xanathars) loaded when you go to create, let's say a fighter. At 3rd level, the fighter gets to choose a subclass. If I've used the drag and drop method of leveling up and I pull over the PHB Fighter, when I get to third level it will show a list of all available sub-classes, regardless of source. So let's say I decide to play a Cavalier out of Xanathar's but I drag and drop the fighter from the PHB, it still shows all of the Xanathar/Tasha sub-class options.

Is there a way to make it so custom entered sub-classes can do the same thing?

So I have entered a fighter sub-class called the Legionnaire, that I'd like to have my players have access to. However, in order for them to actually play the legionnaire that I've manually entered, they must choose that specific mod's Fighter and it never even asks about sub-class.

I hope I've explained this well enough, I know it's kinda out there, but I'm just looking to make it as easy as possible for my players... I'd love for them to be able to drag over the PHB fighter and when they get to select their sub-class have the Legionnaire show up on the list of possibilities. Right now, it acts like it is its own class. I want it to be clear that it's a fighter sub-class.

Thanks

You would need to edit the xml code for the exported module with your class in it. Exported modules use an id number to link things up but the official stuff uses names. So your created class will have something like <id-00012> for an ability whilst the official modules will be <barbarianrage> or similar.

If you unzip the SRD data module you’ll see the xml structure. You’ll need to edit your exported class to mirror that structure.