i got dreadlock extensions and have put dreadlock extensions in others the first day of doing the actual hair. in my opinion it's easier to crochet them in when your dreads aren't matured because that way they can lock up at the same pace and it's easier to crochet the hairs in. i'm actually planning on making a video on how to put them in because i've seen a lot of people ask.
what you have to do is get real human hair close to your color (you can get it on ebay for around $20 and up and make sure it's more than 120g) then when you get it it will be on a weft, o-rings, clip in wefts, whatever. it doesn't matter. just dread the hair then clip off whatever it's attatched to. then after they are dreads crochet them into your real hair and they'll stay put if you do a good job of dreading them into your real hair. :] if you have any other questions feel free to message me