Yes, you can use low-level laser therapy (or LLLT) if you are wearing hair extensions; however, there are several factors to be aware of.
Laser light normally shines through individual strands of hair, easily reaching the scalp beneath. When attaching extensions, the effectiveness of the treatment may be influenced by the opacity of the attachment device. The more opaque the attachment, the more laser light may be blocked.
Another consideration is that wearing thick cornrows or braids attached directly to the scalp can block the laser light's ability to reach the follicles.
As noted in this article on our blog, hair extensions and weaves can actually cause hair loss over time through traction alopecia, which you may wish to consider when selecting this hair style option.