Is there any filter opposite to Polarizing filter, to emphasize reflections?

The answer will be easy to figure out if you understand a little bit what polarization means.

I don't have a polarizing filter to play with, but I do have a physics degree, so here it goes:

Light reflected by certain types of surfaces (such as glass or water, but not metal) is partially linearly polarized. Light reflected under a certain angle is fully polarized.

Linear polarization means that the electromagnetic wave (light) vibrates in a certain plane only, to put it simply. If you rotate the polarization filter to align with this plane, it lets the polarized light through. If you rotate it to orient it 90 degrees to the plane of polarization, it filters it out fully.

Sunlight will contain light of all polarizations, so a polarization filter will only filter "half of it". Reflected light contains more of light polarized in a plane parallel to the reflecting surface: so if you align the polarization filter perpendicular to the reflecting surface, it will filter out more of the reflected light than light coming from elsewhere. If you orient it parallel to the surface, it will filter out less of the reflected light---the effect you are looking for.

So the short answer is: just rotate the polarization filter and find the orientation which makes the reflection look the brightest! This will accentuate the reflections in the photo instead of suppressing them.

EDIT: Here's an extra idea: you could take two photos, one where you minimize the intensity of the reflections and one where you maximize it. Using these two images, you could make the reflection even stronger by subtracting some of the relfection-less image. It'd take some experimentation with an image processing package to see if it is possible to get it right.