Yep, one of the cool things about entropy is that it always increases. If you keep XORing more information into the seed pool, you only ever increase the total entropy.
Also, total entropy is not necessarily the same thing as attacker's uncertainity of the pool's state. The difference seems mostly theoretical, but there are practical issues for both cases of these two quantities beeing wildy different.
Re: Entropy; in a system RNG design, entropy is typically considered relative to an attacker. In some very real sense, the "total entropy" of the system is only the unpredictability of the generator to the most knowledgeable attacker.