I don't know if it'll work or not, but I suppose you could just change the tag to reference your Custom race techtree file. As is the custom race is assigned the Terran techtree, so I assume it is doable.

And i thought this was about using the 'impossible tech' workaround, wrong.
What you really want is TA's tree features and its structural approach, right?
Sic, i too needed some sort of control over a few areas of the 'standard' research_grid (For DA purposes, that is) to insert some Apocalypse concepts straight in between Discovery_Sphere & Techno_Victory... and soon, realized the slots only required tags & preqs to be usable.
Trick was to balance it all out. Lotsa testing yet to do, but i'm slowly getting fair enough results.
If only i had access to the TA editing tools when i planned these alterations, it coulda been a LOT easier.
Bof, it's real fun to fiddle with these issues anyway; in X-Worlds players will get... 4 new Capital buildings, about 40+ new technos, plenty of extra Trade-Goods, a whole bunch of X-Com specific weaponry pathways, possibly three all new pseudo-indirect Victory conditions, etc, etc.

My Arnor-CustomRace plays quite well with its unique techtree and buildings. Some more balancing, some new starbase modules to add and I may upload it.
I'm setting up some ethical aligned techs at the moment--its pure fun, thanks stardock

As soon as it's on the library i will give it try... assuming it's for DA?
If it is, I can see what you've done, and do tech trees for my starwars/bsg/stargate personal mod.

That's a shame.
I have no way of buying TA, so I guess i'll have to miss out.
Still, it looks good.

Presumably not many, certainly not custom tech trees. Custom races made in-game should be fine though, as per DA.