there are a few lens attachments (fisheye and pinhole) that I would love to have, but my one lens doesn't have threads that I could use to attach the fisheye/pinhole. Is there any way I could still attach these, or would it just be worth buying a whole lens? The camera I use is a Nikon D3000, and currently I'm using the lens it came with the most, 18mm-55mm, (which is the one that doesn't have threads) but I also have an AF-S Nikkor-Zoom 55mm-200mm that HAS threads, but I don't use that one nearly as much. Any help would be greatly appreciated. Thank you.
The kit D3000 comes with the 18-55mm f/3.5-5.6G AF-S DX VR Nikkor Zoom Lens I believe. According to the Nikon website it accepts 52mm screw-on filters. You should have no problem adding 52mm screw on filters or attachments such as macro filters or wide angle lenses(example).
If you have multiple lenses that you would like these attachments to work with, you can get step up/step down rings that let you use other thread sizes together. For example you could get a step up from a 52mm filter to a 77mm filter and use 77mm attachments on multiple lenses.
Note that these lens attachments many times come with severe trade-offs as far as quality goes, but they are very inexpensive typically. The other option would be to get a dedicated lens for fisheye or macro work.
There are some clip on attachments. I have a Raynox dcr-250 which is a clip on macro adapter, quality is undoubtedly less than a dedicated macro lens but then the price is also considerably less and its "good enough".
You will need to worry about clearance between the attachment and your lens, I bought it for my Nikon P7100 along with a Polaroid adapter tube which extends just beyond maximum lens reach so that macro lens and camera lens do not collide.