As of update 48.1 we cannot use the Tardis to travel TO nor IN a modded dimension such as Twilight Forest, this was, if i remember correctly, possible in 1.7.10 where there was no coordinate travel.

If there is a way I can implement a custom dimension/planet and is not too complicated, then i'd be happy to hear it.

Step 1: Go to the modded dimension you need to go

Step 2: Summon your Tardis there

Step 3: Go in to your Tardis

Step 4: Go to the monitor and add a new waypoint

Step 5: Now you can go to that dimension when ever you want

Step 6: Repeat with other modded dimensions you have

 That's the first thing I did when I got to a modded dimension. The problem is we can't travel INSIDE the dimension, like if we use Journey map to find coordinates and then type them into the monitor, it won't "find" the coordinates as it's not familiar with the dimension for some reason. The part where we can't travel TO the modded dimension is referring to that we can't find it in the planet selection.

Maybe you could Suggest a Feature for the Mod that would allow the Tardis to Register the Custom Dimensions of Other Modes you have installed and then allow the Tardis to be Able to Travel there by the Planet Selection and Coordinates .. like it does with the Dimension in game already … 

I don't know about the Coding but it possibly Could be done …. (I know nothing about Coding/ Programming so please don't Quote me on this)