Can I use a fountain pen tip as a dip pen? It's for writing it in general. I do not own a dip pen yet, I have water based ink to use.
No. As there is no place for the fountain pen tip to hold a reservoir of ink through dipping.
A dip pen has a slight concave curvature. This curvature helps to hold, with the help of the surface tension of the ink, a small supply of ink on the backside of the nib.
As you draw the dip pen across the surface of the paper the tip of the pen deposits a small amount of ink onto the surface of the paper. The surface tension of the ink and gravity pull more ink from the supply as the pen is dragged along the surface of the paper.
With the fountain pen there is usually a plastic tunnel at the rear of the pen nib which is the tube through which ink in the storage cartridge is supplied to the pen tip. There is no room for the fountain pen tip to stored any usable amount of ink. Dipping the fountain pen into a pot of ink will cause a glob of ink to form at the tip.
If you really really want to try it then have a cloth handy. After dipping the fountain pen into the ink pot wipe the convex surface of the fountain pen tip after dipping. The nib will have a bead of ink on it. And you want to get rid of this bead before touching the tip to the surface of your paper. After wipping the drip away. There will be a small amount of ink that will remain on the exposed concave surface at the back of the pen. All depends on how far the plastic tunnel covers the back of your fountain pen nib. It will be a small supply of ink but maybe it will be enough for you to try. You will have to dip and wipe often as the ink will run out quickly.
It depends on the type of fountain pen nib, but yes, you can. Obviously, you want the right tools, know-how, experience, and touch if you plan to dismantle a pen, and it's all at your own risk. Marshall & Oldfield's Pen Repair is a good guide on how fountain pens go together and come apart if you're completely new to this.
With some pens, you can just wiggle the nib out of the feed/section. Most nibs are simply friction-fit. Moderns, however, may not be, so be careful.
With a vintage fountain pen, the easiest way to remove the nib is to remove the section from the barrel (heating and a gentle touch will keep you from cracking the plastic/hard rubber). Some can be cemented in with a form of resin, so heating will soften the resin--obviously, you want to control the heat so you don't melt/deform the plastic of the barrel/section.
Then, remove any feed tubes (also usually friction fit, but if this is a modern calligraphy/art type fountain pen rather than a writing one, it could be one piece), soak to make sure dried ink isn't gluing everything together, and then either tap the feed+nib out from the back of the section (a wooden nib block with a hole drilled in it big enough for the nib/feed but small enough to hold back the "shoulders" of the feed can be handy for this) or, you can just pull the feed and nib out of the section with your fingers.
Once removed, the nib can then be inserted into an artist dip-pen holder (if you have one).
If you don't have one, then removing the nib is unnecessary, and you can just dip the fountain pen in the ink--as long as it's water-based and can be dissolved by water, you're unlikely to do any damage to the fountain pen, since soaking can remove the ink should it dry and gum up the feed channels. I would not, however, recommend using anything like india ink which will destroy a fountain pen.
Fountain pens evolved from dip pens, and the basic nib is still pretty much the same: a piece of metal that comes to a point that is tipped for smooth writing, with a slit for capillary action to take the ink and feed it in a regular manner to the paper. The only difference with a fountain pen nib is that it's designed to fit against a feed to regulate the ink flow from the ink storage.
Obviously, if your fountain pen nib is, say glass or a stylo, however, no, it won't work to remove it and try and place it in a dip-pen holder. :) :) The glass nib can also be used in situ in the fountain pen as a dip pen (and will probably hold just as much ink a glass dip pen), but stylos are stylos and they are fountain-pen only.