Dear Mary ,
Thanks for contacting Top China Travel!
We are professional agency in doing inbound tourism, therefore we can't not add on side trips to Bangkok and Siem Reap. But We are happy to help you with your trip in China, later we will send you the details of itinerary and the price.
Would you mind to advise me:
1) When will you plan to arrive?
2) Which star hotel do you prefer to stay.
Any further request or question, please feel free to contact with us.