You can but the result will be poor.

First of all you need to disable the lighting from below, otherwise what you will get will be the reflection of the surface.

Second you need to light the negatives from behind with some source of light that is both known and homogeneous: you don't want to have one side of the negative brighter than the other one.

Third you need a scanner with a good optics to focus at some millimeters from the glass surface: usually holders don't place the film in contact with the glass.

The best solution is to buy a slide/negative module for your scanner: it will take care of turning off the light below and to light the film homogeneously.

Still, don't expect sharp results: as a matter of fact, the attainable resolution ("effective", the details you get, not the pixels you save) is about 1000 dpi, even with 2800 dpi scanners (Epson Vxxx). This means a photo will have details corresponding to 1500*1000 pixels: 1.5 Mpx.

If you don't want to buy anything, disassemble the scanner, turn off the lower light (or, even better, connect a switch and bring it outside), then place a very powerful light (100W halogen) at about 50 cm distance (so that it is more or less uniform), do the scan.

But I don't think it will work, because if the light below is off, the circuitry will notice it during calibration (it is performed just before each scan) and an error will be generated. That is why I said bring outside a switch: after the scanner gets out of its hiding position and is visible under the glass, calibration is done and you can turn off the light below.

Still, it won't work: the calibration will bring data from the lower light, that is much stronger and "whiter" than your 100W halogen bulb: the images will be dark.

I think in this case there is nothing you can do for free with a flatbed scanner.

MUCH better to build this: http://petapixel.com/2012/05/18/how-to-scan-film-negatives-with-a-dslr/ is it an adpater (home made) that allows a (APS-C) dSLR camera (with a proper macro lens!) to be used as negative scanner. The results will be after some tuning very probably better than a medium range flatbed scanner (200-300 Euro/dollars) and WAY better than anything you can achieve with yours. Of course, you need the dSLR and the macro objective.

I suggest the last option: dSLR. A 1" camera (Sony RX-100) is also fine, or a 4/3 camera, but anything smaller than that should not be used. I mean, the time you spend would not be worth the result and if you take the effort, do it well.