How many subpanel breakers can a single piece of main breaker/bus support?
In the US, part of the panels ratings are the maximum current for a single bus stab.  Many panels say on the label not to exceed a certain current level on a single bus stab.  Most residential panels are double row so a single bus stab feeds two breakers, one on the left, one on the right.  The sum current these breakers supply can't exceed the bus stab rating.  

There are other ways to feed a subpanel.  If your panel has or can be fitted for feed through lugs, you can feed the subpanel from those lugs.  

The NEC has feeder tap rules that allow you to tap the feeders for a subpanel without overcurrent protection - normally a big no-no - within certain limits.  The feeder tap rules outline these limits - distance limits, running in conduit, etc.  

Another option may be to change out the panel "guts" with a retrofit interior kit.  With these kits you remove and replace the bus, lugs, etc. If Siemens makes a retrofit kit with a bus compatible with your breakers, you could replace the guts and use your Siemens breakers.  However you'd have to replace all the breakers in the panel with Siemens breakers as well.