Thanks so much Daniel! It works as it loads the texture outside the atlas, however, I had an issue with other textures not being displayed.
It was because I just wanted to have the atlas textures as a backup, it means that when there exists another texture (outside an atlas) with the same name, I needed to get that one. If there not exist another texture (outside an atlas) I needed to get the one from the atlas.
Anyway thanks to your suggestion I was able to do what I needed, this way:
var t1:Texture = assetsModel.assets.getAsset(AssetType.TEXTURE, _textureName, true) as Texture;
var t2:Texture = assetsModel.assets.getAsset(AssetType.TEXTURE, _textureName, false) as Texture;
return t2?t2:t1;