As the cogs in cassette seem separable, can it be converted into single speed by just putting the 9 identical cogs of the gear you like and retaining the rest of the gear mechanism as it was?
Such a cassette should theoretically last much longer, as you simply shift to another cog when one is worn. It may also be lighter if the weight of the selected cog is below the average.
Since 1982 or so just about all rear derailleurs are built with a slant parallelogram design so that it glides in a diagonal line as it shifts up and down. This is supposed to keep the jockey wheel a constant distance from your cogs, which are arranged from biggest to smallest. It is a very popular design because it just works really well and the patent lapsed in '82.
If all your cogs are the same size, as you shift out towards the outside of the bike your derailleur tends to get closer to the cogs (since they are all the same size).
SO: To make this work well you will probably need to find a rear derailleur that is not a slant parallelogram. These exist in some very cheap designs. I think this hack is absolutely worth it, if only for the lolz.
In order to get all those cogs you may have to cut up old used cassettes. You can buy separate cogs using Miche system as well but it will be more expensive.
I would say : no way.
The rear derailleur is not supposed to handle cogs with the same dimension. The cogs should be smaller towards the outer part of the cassette, to correspond to the movement of the RD.
Additionally, you'd have to have at least one different cog, probably an 11 or 12 tooth outermost, because it has a differnet design to every other cog.
This little cog also has the detents for the lockring to engage into, and without them the lockring could back itself out letting all the cogs flop around somewhat. This would give sketchy riding at best.
Also, a singlespeed rewards a good chainline. That means the chain goes straight-into/out of the chainring and cog. By being out of line, you're wasting power, adding wear, and removing the chance to use chunky 1/8" chain instead of flexy 3/32" chain like derailleur bikes have to use.
It would still be a 9-speed. Each of the 9 would be 9 instances of the same speed.
You can get spacers for installing a single sprocket on your freehub. Also, rvil76 makes an important point about shifting.
It may work, but why would you? Yes,technically it would be a single speed, but the reason for having a single speed is, as far as I know, to have a bike that is as light and simple as possible. By making a "same speed" rear cassette you would get (at least nearly) the weight of a 9 speed and all the complexity of a 9 speed without the flexibility a multi speed bike gives. So this would give you the worst of both parts.
On the other hand, if this was something you thought about doing just for the joy of doing something weird, just feel free do do it and probably learn a few things on the way,
No.
Primarily because most cassettes are not actually fully made of separate cogs and the top few are usually a single piece.
I think the closest you'll get is to just have a cassette comprising the biggest three cogs (which are usually joined), and then the rest of the space for the cassette filled with spacers.
The photos of this hill climbing bike demonstrate it: https://webcache.googleusercontent.com/search?q=cache:mgm6Li4DdToJ:https://cyclingtips.com/2020/08/6-2-kg-three-gears-and-cut-off-drops-the-bike-used-for-the-everesting-record/+&cd=11&hl=en&ct=clnk&gl=uk (you may have to scroll down as the top of the web page seems to be a big blank white space).
"[This time] I removed most of the rear cassette just leaving the 25-, 28- and 32-tooth sprockets.” The rest of the freehub body was filled with aluminium spacers commonly used for single-speed conversions."
I don't know why you'd want to do this (except for in extreme circumstances such as wanting a bike to set a hill-climbing record!).
Just adding another reason why this may not work.
All cogs sit outside the vertical plane of the wheel. Any force you apply to the cassette will (very slightly) bend the axle.
That force has most effect on the cog that is farthest from the wheel. But as that is usually the smallest cog, that effect is reduced.
When you replace the farthest cog by a much larger one you risk bending the axle.
