Edit: The question now says "millions of new characters". This makes it easy to answer:
No. Utf-8 is a Unicode encoding. Unicode have a codespace which allows 1,114,112 distinct codepoints, and less than a million is currently unassigned. So it is not possible to support millions of new characters in Unicode. By definition no Unicode encoding can support more characters than what is defined by Unicode. (Of course you can cheat by encoding a level further - any kind of data can be represented by just two characters after all.)
To answer the original question:
Unicode does not support languages as such, it supports characters - symbols used to represent language in written form.
Not all human languages have a written representation, so not all human languages can be supported by Unicode. Furthermore many animals communicate but don't have a written language. Whales for example have a form of communication which is complex enough to call a language, but does not have any written form (and cannot be captured by existing phonetic notation either). So not even all languages on earth can be supported by Unicode.
Even worse is something like the language of bees. Not only does it not have a written form, it cannot meaningfully be represented in written form. The language is a kind of dance which basically points in a direction but relies on the current position of the sun. Therefore the dance only have informational value at the particular place and time where it is performed. A symbolic or textual representation would have to include information (location, position of the sun) which the language of bees currently cannot express.
Even a written or symbolic form of communication might not be possible to represent in Unicode. For example illustrations or wordless comics cannot be supported by Unicode since the set of glyphs is not finite. You will notice a lot of pictorial communication in international settings like an airport, so it is not inconceivable that a race of space-travelling aliens will have evolved to use a pictorial language.
Even if an alien race had a language with a writing system with a finite set of symbols, this system might not be possible to support in Unicode. Unicode expects writing to be a linear sequence of symbols. Music notation is an example of a writing system which cannot be fully represented in Unicode, because meaning is encoded in both choice of symbols and vertical and horizontal placement. (Unicode does support individual musical symbols, but cannot encode a score.) An alien race which communicated using polyphonic music (not uncommon) or a channel of communication of similar complexity, might very well have a writing system looking like an orchestral score, and Unicode cannot support this.
But lets for the sake of argument assume that all languages, even alien languages, can be expressed as a linear sequence of symbols selected from a finite set. Is Unicode big enough for an alien invasion? Unicode have currently less than a million unassigned codepoints. The Chinese language contains a hundred thousands characters according to the most comprehensive Chinese dictionary (not all of them are currently supported by Unicode as distinct characters). So only ten languages with the complexity of Chinese would use up all of Unicode. On earth we have hundreds of distinct writing systems, but luckily most are alphabetical rather than ideographical and therefore contains a small number of character. If all written languages used ideograms like Chinese, Unicode would not even be big enough for earth. The use of alphabets is derived from speech which only uses a limited number of phonemes, but that is particular for human physiology. So even a single alien planet with only a dozen of ideographical writing systems might exceed what Unicode can support. Now consider if that this alien already have invaded other planets before earth and included their writing systems in the set of characters which have to be supported.
Expansion or modification of current encodings, or introduction of new encodings will not solve this, since the limitation is in the number of code points supported by Unicode.
So the answer is most likely no.