Hi,
In the MSMQ MP, the number of messages in queues is monitored using Self Tuning Threshold monitors, so you cannot change the threshold >> check the MP Guide for details:
http://www.microsoft.com/download/en/details.aspx?id=12962
If you need to create your own monitor, then you can create a Monitor - Windows Performance Counter >>
Object: MSMQ Service
Counter: Messages in queue
OR > Total messages in all queues
Hope this helps..
Regards, Mazen Ahmed