Of course, but there will still be the problem that the people downliading there won't know the content. But if the game is slow paced enough that might not be a problem at all.
Also, if user generated content would be parted into groups for how much you need to be familiar with it to use it.
If the unit creation system it intuitive enough then adding different kinds of mounts wouldn't be that hard to learn on the fly. Like when you find an animl resource, like a horse, the horse resource has certain walues, like "mount" and speed, attack, def and so on. And if you then suddenly add bears as a user content it will also be a "mount" but with different speed attack def etc. And could be used in the same context as a horse, but with different results.
That kind of user content would be rather intuitive and blend in enough that the player wouldn't have to learn a whole new strategy. But if you have more complex content, like new buildings, or new resources with a whole wariety of attributes, like suddenly you have a new type of metal that give a whole range of abilities to armour and weapons it may get too complex for the user to learn on the fly.
Of course, this is only speculations, as we don't know how fast paced the game is going to be or how the reasources will have attributes or not, or what kind of user generated content will be posible.