Can you use the internal O2 Concentrator and bleed-in additional low-pressure oxygen at the same time?

Yes. Oxygen via bleed-in from an external low-pressure source is additive, and can be used in conjunction with the internal O2 Concentrator or external high-pressure source. 

If you are using the internal O2 Concentrator and want to add additional oxygen via bleed-in from an external, low-pressure source, leave the internal O2 Concentrator running (as shown on the Oxygen therapy screen), and then simply add the low pressure source to the back of VOCSN and turn the oxygen source on.