The depth of field is a function of the relationship between the image magnification and the diaphragm opening (aperture).
You will have to reduce the aperture at that magnification or reduce the magnification at that aperture.
Changing the lens focal length to affect d.o.f. from a given subject-camera distance is changing the image magnification, in effect.
** One way to reduce the aperture, if the microscope has a fixed aperture, is to use a "field-stop."
To make a field stop, cut/or punch a clean circular hole in an opaque (black is better) stiff paper or thin card. You can use a sharpie, or india ink to make an index card black enough, too. Maybe you can find the right-sized metal or fibre washer in a hardware store or misc. parts you may already have around your studio, or lab, shop or house - paint it black to cut down flare. Tape it in place over the lens.
Place your aperture in front of the microscope lens centering the aperture. For some gross-specimen low-magnification 'scopes, this is relatively easy since the lens is quite large.
The field-stop acts as an outboard aperture to limit the light entering the lens to the centre. The effect is increased apparent depth due to the "stopping down" (reducing the aperture) of the lens.
Experiment to find the right aperture to achieve the depth of field you wish. The exposure will have to compensate for the reduction of light. There might be some vignetting, but c'est la vie.
The background could be a bit lighter, too, which might help the image detail rendition.