Hi Stefan,
the non-B&O sub has to be connected in parallel. Be sure that you buy an active sub. The connection is described in the reply above. You can connect left AND right channel to your sub if it has a stereo input.
You can not run the beolab speakers through a non-B&O sub. In general this shouldn´t be necessary as the sub is only reproducing low frequencies and the beolab speakers are only reproducing "higher" frequencies depending on their specifications.
When you look for a sub, you should pay attention to its adjustments. It should provide 3 settings:
1. Volume
2. Crossover frequency (to determine, up to which frequency the sub shall work; also to avoid that certain frequencies are reproduced by both the sub and the beolab speakers, which would result in an undesired increase of volume at these frequencies).
3. Phase (to compensate the distance between the sub and the beolab speakers. Cheap models provide 0 or 180 degrees, better models provide a continuously variable adjustment.
As stated already above, you will need an active sub. Look for a sub that is made for HiFi rather than for video purposes as the first are more precise and the latter only produce "rumbling"...