It is important to note that DLC are just weapon skins; therefore, they do not have their own stats. Furthermore, it is up to you to decide which weapon you want to overlap with the DLC of your choice. 

Use this as a reference:
http://monsterhunter.wikia.com/wiki/MH3:_Weapon_Gallery

Rename the weapon of your choice to whatever corresponds to what you want to use it for. 

Also, DO NOT rename the weapons and just leave it in the "weapon_dlc" folder because it needs to be in either:

weapons_E for US files

or

weapons_P for EU files

Obviously, you're going to have to delete something if it already consumes a slot. Just redownload the package to get the file back or move it outside the folder if you want to ever use the old file again.

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More DLC coming soon?