afaik, this is not possible currently with the 2.0.x versions
but in the master branch it looks this is possible.
I am not sure when Tribe29 is going to release this, perhaps in the 2.1 ?
Should be possible to port this to 2.0 by putting the needed files in the ~/local folders
and removing the old haproxy check which is still using the old check api.
See:
checkmk/haproxy.py at master · tribe29/checkmk · GitHub line 51ff