It would be easier if the S90 incorporated a mount for an accessory tube, like the LX3 does. But if you have access to a lathe you can machine a simple adapter, as shown below, which slips over the S90 innermost lens barrel. I used the adapter tube from my LX3, which has 46mm filter threads.
The insert in the LX3 tube is Delrin (acetyl), a very easily-machined plastic. It does not scratch the lens barrel when it is slid on or off. Here is a polarizing filter attached, probably the only filter you will want to mount. The tube does not cause any vignetting, even at 28mm.
Because Delrin has such a high C.T.E. temperature changes cause the Delrin insert to expand or contract at ten times the rate of the aluminum adapter tube. A hand-sawn slit takes up this slack and makes the device usable over a range of temperatures. I've tested it at 40ºF (in my refrigerator) and 80ºF (the current temperature here in Phoenix), and at both extremes the fit is not too tight or too loose.
It's also possible to mount high-quality close-up lenses, such as those from Raynox. Here is a full-frame 28mm f2 shot without a close-up lens. As you know, the close-up range is restricted to the wide end, near 28mm, and the working distance is between one and two inches.
Here is a full-frame 105mm f4.9 shot with a 5.0 diopter doublet close-up lens. The working distance increases to four inches or more, depending on the magnification desired.