Seems crazy to add sub-trees, but it is often talked about. I figured out a plausible method for adding crews for these subtrees.
How about if for every full point you place into the main country, a half point is earned in that subtree as if the bigger country trained the smaller country. If you play that subtree, you put full points into those crews and not the host country. This way if you have already grinded out the main country, the subtree will have some partial skill for the crews.
Sounds crazy, huh?