I was just wondering if there was an easy way to add items from one loot table, like crate_normal, to different one, like crate_normal_2 etc. My endgoal is to have things like roadsign jackets, kilts and some other items appearing in crate_normal_2 in addition to the normal crate_normal_2 loot, but without using rarity tables.

This is as of right now not supported. If you are looking for a more in depth looting system, check out Loot Plus (on uMod)