Yes. If a MOBI is DRM-free you can side-load it via USB or e-mail it to Amazon for Whispernet delivery (for a charge) or WiFi delivery (free). Any MOBI you get from an author's site or via e-mail with be DRM-free. Some publishers also provide DRM-free ebooks, e.g. Baen's webscriptions. See, for example,
Using the Amazon Kindle with WebScriptions.