"Overseas", as far as I am concerned, is an adjective or an adverb. If "from overseas" is a correct phrase, why is it grammatical? "From" is a preposition, and it should be followed by a noun, not an adjective, nor an adverb.
Firstly, "overseas" can be used as a noun:
I am assuming you mean something akin to "It came from overseas." This form of "from" is sometimes used without an explicit noun:
In this case, the noun is actually something like "here" or "there":
If you really want to think about "overseas" as an adverb, then expanding it would look something like this:
But that the extra "from here" is implied and can be dropped without changing the meaning.
For what it is worth, if you search for common usage of adverbs following "from" you'll find that most of them can be used as nouns:
"Overseas" is just another word in that list. This particular type of word is used to denote location and, as such, tends to get smushed in whenever we need to refer to a location.
It would be an interesting question to compare historical usage of these words in order to determine whether they slowly become accepted as nouns over time. But that would be more suitable as a separate question.
Structures as The cat came out from under the couch can be made plausible by inserting a noun as in The cat came out from her place/hiding place under the couch.
from overseas: from a country/somewhere overseas.