That I can add a .mpq file to a map and have eud actions load it somehow? If yes, how?

EUD actions require a mod in the first place, so that somewhat defeats the purpose.

If I understand what you're asking correctly, SCMLoader should do the trick.

But if you're talking about just sticking an mpq archive into a map and then trying to load custom files from it in-game, no, that's not possible. At least, not without a ridiculous amount of work.

What you mean EUD actions require a mod in the first place?

You cannot run EUD actions without modifying your running game instance in some way. Whether this is by downgrading to a version that supports EUD actions or using a program such as Farty's Action Enabler, it requires a modification, or a mod for short.

>.> I already knew that
How would I get EUD Actions to load the mpq file though?

What are you trying to accomplish, exactly? There may be a much better way to do this.

As far as I know, there hasn't been extensive testing to perform such an action, so we can't just say, "Oh, make a trigger pointing to death 13333337 and it will load your MPQ." EUDs are memory overflows; you can read memory addresses using EUD conditions, and you can write to memory addresses using EUD actions. I highly doubt these buffer overflows could pour into a separate file that you put inside the map.

I recall hearing that one of the reasons Blizzard eliminated EUD actions was for the potential of spreading viruses that were packed into a map. Whether this is true or not is beyond me, but I believe the primary reason was because it was causing maps to crash and Mac players to desync with PC users.

Finally, you wouldn't be able to do anything with this MPQ that would affect the game, even if you could point to it using EUDs.

Also, in the future, make your topic titles more appropriate. "Loading an MPQ using EUDs" is much better than "Is it possible...."

To elaborate on this...

The problem is to access the mpq which, for the moment, lies dormant on the players hdd. There's no "built-in" action to activate the mpq so you'd have to write code that accesses the users hdd and reads whatever value you want to change and use EUD actions to write it to the respective part in memory. This is essentially a virus.
The alternative would be to store every byte that needs changing in triggers. I'm not sure if the addresses are researched or if it's even possible if you want to change graphics or sound.
In any case it's just impractical.

I guess that will be a lot of work
Hopefully, someone will merge EUD Action enabler with scmloader together one day

Snippet from SCMLoader changelog:

SCMLoader does support FireGraft changes, and since the EUD Enabler is available as a FireGraft plugin, you could try that. I'm not sure whether SCMLoader will actually load the plugin though; AFAIK it only loads actual FireGraft changes and not plugins, but it's worth a shot, so go ahead and try it.