Any strategies or tips on how to make juice into creams for filling cakes?
There are many thickeners that can make fruit juices into thick fillings. 

You can make a traditional egg custard, as bob1's reply to your answer mentions.

You can make a non-egg filling using other thickeners, like gelatin or starches. Popular starch thickeners to use for pastry fillings are corn starch and tapioca powder. Any of these will work well with liquid flavoring ingredients like fruit juice. Corn starch is very cheap, at least here in the U.S.

Yams already contain a lot of starch and very little liquid, so you will need to add liquid (milk or cream should work well) if you plan on using additional thickeners (which may help achieve a smoother, less chalky texture). I do not recommend gelatin for yam filling because I don't believe the textures will go well together.

"Yam" is a word that is applied to a bunch of different roots, but I think it's worth looking at this Portuguese Trutas recipe. These sweet potato pastries are amazing and don't require additional thickeners. Note that the yam or potato should be pureed, not mashed.