You could try to use it as a centre speaker (with a mix of left&right) to stabilize the stereo image.
If you routing specific notes/ranks to a single separate speaker, the stereo information will be losts for these notes/ranks. I would not recommend that except for bass frequencies but for that you have a sub already.
Another option is to use it as supplementary "subwoofer". Multiple subwoofers in different corners of the room are often recommended because peaks and nulls in the bass response can be averaged out to some extent. Because the ULL110 is a full range speaker, it might perform reasonably well in the area down to 60Hz or so (don't know, speaker measurement data appear to be unavailable).