Edit the appropriate guard leveled list. To find out which one, open a few of the Imperial Guards' Actor Values in the CK.
I'll walk you through one and do it right now. Just give me a bit to write a guide

Okay so I'm going to open a guard's Actor Value...I'm randomly picking in this case:
CWSoldierImperialNotGuard, I"m guessing he's an Imperial Soldier who isn't a formal Guard.
I'm not 100% sure if he's what you're looking for but this will be good for a general example

.
So I open him up and click
"Full" at which I see a big... "M" sign. This M I believe means that the character has no "identity", in particular this character has no identitiy because of something in the corner of his actor window.
You should see
"Template Data" with a few things checked:
Use Traits, Use Stats, Use Scripts, Use AI Data, Use Inventory...etc. Almost everything is checked here. What this means is that this particular record "
CWSoldierImperialNotGuard" uses all of the checked traits of the
"ActorBase" value shown, instead of having its own stuff. In this case the
ActorBase this record uses is
"LCharSoldierImperial".
You want to change the guard's inventory though...so we have to find an editable inventory record, and if possible, the base record (the one that everyone is referring to).
So this particular record isn't much use to us. Let's click
Edit to see where
"LCharSoldierImperial" takes us.
When you press
Edit it will bring up a
"LeveledCharacter" list. This is a new feature in the CK (I believe at least). Basically some records point to a list of customized characters from a list. This way they can have templates of characters stacked up without having to hand place everything. The same goes for the
Template section I explained above.
But anyway lets pick anyone from the list...lets say
"EncSoldierImperialBretonM01MaleSoldier"After opening
"EncSoldierImperialBretonM01MaleSoldier " Click Full to look at the Soldier. You'll see a nice mustached Breton Imperial Character. Cool stuff! Definitely a bit closer to editing a record with the proper inventory.
But if you notice in the bottom left his
"TemplateData" has "
Use Inventory" referring to the inventory of another record...dammit! Gotta dig deeper.
Click
edit to open this record:
"EncSoldierImperialTemplate".Now that you're in
"EncSoldierImperialTemplate" you'll notice that this particular record doesn't use any template data. This means that this is the base record. Awesome! You can click
"Full" to see what it looks like. It's basically a plain Imperial NPC.
But we don't care about his looks for this one, you want to Edit his Inventory! So click the
Inventory tab for the NPC.
You'll notice he uses a specific Inventory item called
"CWSoldierImperialGear". It's icon is a green question mark...which means its a
Leveled List, or basically a list of items that will be called upon at random when its used.
You'll also notice that he has some
"Outfit Objects". Outfit is basically the particular clothes/armor that the NPC is FORCED to wear. There are two of them: Default Outfits and Sleep Outfits. You can edit these later on if you desire, but we'll focus on editing his weapons for now.
So double click
"CWSoldierImperialGear" to open up the Leveled List for it.
You'll notice that he uses some weapons in particular:
ImperialBow, ImperialSword, SteelDaggerAlong with some other items at random:
LitemTorch50, BaseArrowSteel75, LootDrinkList25, etc etc.You can open up those leveled lists to see what they contain too. But the main thing to pay attention to is the weapons he's equipping.
If you want to add your weapon to all the guards you'll have to edit this
Leveled List. You can choose to remove
"imperial sword" and drag and drop your weapon into this list (after you make it as a new weapon record of course!), press ok and then your weapon will move out to all NPCs who use this template.
That's pretty much the extent of it!