If you want to extend them, extend them.
When i got my dreads i had to have them long enough to tie back neatly for my job, so i extended some of them with real human hair.
Despite what Soaring Eagle says about them never feeling like part of you, that's just how HE would feel about it, you are your own mind.
If you extend them and don't like them, you can always take them back out and carry on normaly.
I always planned to cut the extensions out of mine when they grew long enough, but when they had i found i was unwilling and kept putting it off and off, until i realised i can't remember which ones were extended anyway. I've chopped some of the very tips off, but i can't bring myself to cut them properly, it's been my hair for nearly 3years, it's part of me now, and just as much a part of my dreads.