i did the 2nd mission of campaign you get a item from both missions. (2-1 and 2-2) one of the items is called "leader attachment"
go to dress up and power up
go into the item modification right bumper till you get to accesory
click on loader attachment
click down to empty slot
click on the other item you get from campaign
you will need 1200 happy stars Woody2211 says you need 1600 happy stars
edit:
it looks like you cant remove the orange buff
you can put on the white buffs or remove the white buff altogether
purple are bad buffs that you can remove for star points
you can only buff like items: shields and shields, armor and armor, and so forth with the random items you get you may be able to do this sooner than the second mission
Edit:
example:
item 1
green skull shield
guard power 238 (0)
anti magic guard 245 (0)
[empty slot]
item 2
modern shield
guard power 182 (+92)
anti magic guard 214 (0)
guard power lv1 <~~ white lettering
click green skull shield
click empty slot
click modern shield
click guard power lv1
you will need happy stars to do this or the happy tickets to complete
Note: you cannot use the different class weapons you cant use a knight weapon and a mage weapon or the cleric weapon is it has to be the same class weapon