You appear to be confused about terms like zoom and focal length. Focal length is the distance between the lens and the focal point of a parallel beam (i.e. focused at infinity). To focus it closer, you increase the distance between the sensor and the lens. To produce an image on the sensor at the same size as the object, you would need a distance between lens and sensor of 400 mm and a distance between lens and object (working distance) of 400 mm. This assumes ideal thin lenses, things get more complicated with objectives consisting of multiple elements, as used in microscopes. Zoom is the ratio between shortest and longest focal lengths, and only applies to zoom objectives. A single lens has a zoom of 1, since it's fixed focal length.
You could mount a lens in front of a webcam, but I doubt whether you'd get anywhere close to microscopy-level magnification. You would need a fairly short focal length (high curvature) to have any effect. Focal lengths add up as the reciprocals, so 1/f_t = 1/f_1 + 1/f_2. The unit of reciprocal focal length is diopter. If the focal length of the webcam lens is 4 mm (250 diopter), then you would need approximately another 250 diopter to get it down to lifesize (image size = object size) magnification, at an impractical working distance of 4 mm. Increasing the working distance would require increasing the distance between sensor and lens or more complex arrangements like relay lenses. Corner quality is probably going to suck for a single element lens, which is not exactly optimized for performance at 4 mm.
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