Weapons/Clothing are added to the player's inventory after a mission via the Unlockable(s) awarded after the mission, which is governed by the All About Unlockables thread that Matt linked previously.
If I remember correctly, there is no clothing in SRTT that gets added to the store explicitly via some Unlockable. Rather, the unlockables simply immediately add it to the player wardrobe.
However, the store contents are populated via some code that includes looping through all Unlockables that exist and, if they've been awarded to the player, adding any referenced clothing/outfits to the store interface.
In other words, if there's an Unlockable for your clothing/outfits, and you find some way to award it to the player, it will be in the store. Unfortunately, I don't think there's a way to put it into the stores without ALSO granting it to the player directly.
I'm pretty sure gated weapons work just about the same way.