One of the issues I always had with D&D rules is the restrictions on skills. Feats I can let slide, as it differentiates combat styles and prerequisites make sense a lot of the time. Don't want spell focus if you can't cast spells. But class skills are kind of just arbitrarily chosen for what someone THINKS a fighter should be, or a monk, or whatever. And I think it'd be nice to open that up to choice.

So I was wondering, are there any mods that make all skills class skills for all classes. Or is there a way to modify files to make it happen?

Which Game?

I don't know of any mods, but in NWN you could change your skills with Leto 1.69
And NWN2 with a GFFEditor, where the skills are:
1: Concentration
2: Disable Device
3: Nothing
4: Heal
5: Hide
6: Listen
7: Lore
8: Move Silently
9: Open Lock
10: Parry
11: Perform
12: Diplomacy
13: Sleight of Hand
14: Search
15: Set Trap
16: Spell Craft
17: Spot
18: Taunt
19: Use Magic Device
20: Appraise
21: Tumble
22: Craft Trap
23: Bluff
24: Intimidate
25: Craft Armour
26: Craft Weapon
27: Craft Alchemy
28: Nothing
29: Survival

NWN1 is the game. NWN2 has the able learner feat that... mostly... takes care of it. I'm a little more ok with that. Though being able to change it in both would be great.

And yeah, I know how to mod a character with a gff editor, but I'm wondering if there's a way to alter the classes themselves so I don't have to. I mean... if you want to progress normally rather than give yourself a ridiculous edge it's kind of a pain to do.

If anyone should decide they want a similar mod, I got the answer elsewhere:

Install this
http://neverwintervault.org/patch/nwn1/english/nwn-sou-hotu

Then download this
http://neverwintervault.org/project/nwn1/other/nwn-hordes-underdark-169-full-2da-source

Those 2das will include the cls_skill_* files. Each is a table that can be edited in notepad++. 0 is not a class skill, 1 is. You can mod them all pretty easily with a little copy and paste. Then drop the new files into your override directory.

CLS_CLASSES also has a lot of how the classes work. Seems like you could tweak the classes to make a custom one pretty simply. Haven't tried yet though.

Edit:
Lol. Gaunathor beat me to posting it. Thanks for answering regardless.