All Vanity Hairs are sold with copy and mod permissions. 
Due to the scripts used in the hair you will see, that once in your inventory, your hair says it's no Mod, IGNORE THAT, the hair is FULLY MOD!

That being said, here is a very handful tutorial  i found in my inventory,on how to edit your hair. Unfortunately i dont know by who was it made to give her the credit.

BEFORE YOU DO ANYTHING ELSE...make a copy of your new hair!!! By necessity, hairs are no transfer items which means that they cannot be returned. To copy your hairstyle, right-click on the item in your inventory & select copy. Then right-click on the folder containing the original item & select paste. You may wish to house your originals in one folder & your adjusted styles in another, your choice...

  A bit about hair creation: Hair is normally created on the creator's head &, because individuality in SL is a must, everyone's head is a little different. This is why some styles might need to be adjusted slightly to fit your unique proportions. The steps below will work on hair bought from any designer, so long as they have set their permissions to modify.

~*~*~ To make your hair larger (or smaller), follow along with the picture tutorial:

Special note: If the style you want to adjust is in 2 parts, be sure to select BOTH parts of the style so that both pieces will stretch at the same time & in the same proportion! To do this, select one part of the hair & choose Edit from the pie chart, then hold the shift key down while clicking on the other part of the style...both should now be highlighted...

Pull the little white box up (or down, if you wish your hair to be smaller) as seen in pic6.

~*~*~ Hair *through* your eye or elsewhere:

  Because everyone's shape is different, there are times when a pesky piece of hair sticks out of your eye or through your forehead. You can fix this by following these steps...

Now that everything has been adjusted for your own custom fit, get out there & show off that new head of hair!

SL now gives us the ability to wear several items on the same attachment point. Thank you SL :)
In your inventory, when you right click on an object, it will show you an extra feature :  (add).  Using (add), you add the object to the one you wear. Using (wear) you replace the object you wear..
The addition of multiple objects is valid for all attachment of your avatar 

Hair extensions have been attached to the skull, yes same attachment point as hair.
In order to wear your hair and your extension at the same time, you have to wear your hair first, and then, instead of wearing the hair extension you have to click on "ADD" option. This way you will be able to wear both at the same attachment point.

Please note this wont work in SL oldest viewer, so you need to update in order to benefit from this new feature.