I could be wrong, but...You can by using a JACE and mapping in the points you want via Bacnet from a BCU. I haven't done it yet, but I'm told it's not too hard.
Recently I spoked to past Trane regional sales manager. He's indicating that Trane may need to be hired to map points in the local controller to be available via the BACnet access out of the BCU. Was hoping to avoid using Trane if possible.
qtip is correct, using JDE (if using R2 Tridium) or AX quivalent you can learn the BCU and this will give you all the AI, AO, BO and BI. It is very straight forward. You should only need the Network ID used in the BCU which from memory defaults to 1.
These guys have you on the right track, I have done the same thing on a college campus with a Tridium and Summit 16 system. However, not all points in Summit are created as Bacnet objects, a lot are points in devices without object IDs. For these, you can just create the appropriate (AI, BO, etc) object in the BCU and then Tridium willl pick them up on Bacnet. Let us know how it works!
You will need to create points that have not already been created as AI AO BO BI, THis is due to the proprietary points, they are not broadcast as bacnet points, but instead comm 3, comm 4, or Lon. I would pull points into the Jace and then see if you are getting what You want. You however will need some one with summit access, to create the points and map them in. If there is a summit work station on site you can do this your self. Good luck it is not hard to pull it into the Jace using AX, the Jace sometimes is tricky getting all the settings to take, but once you do it works good.