Can I place a regulator in this way?
It's completely possible to run analog circuits from the same supply as digital circuits. There will be high frequency components noise on the Vdd and the return due to the switching. I don't know where you get the idea from that the supply current of the MCU will have only low frequency components. 

If your requirements include very low noise or high performance analog circuitry, then you have to add another regulator, filtering or whatever is required to meet the system specifications. 

It's not unusual for a switching regulator to have tens of mV noise on the output. Generally that's not a problem for digital circuits, but could be a complete non-starter with a sensitive analog front end. On the other hand, if your analog requirements are low frequency and 8-10 bits in accuracy, it may not be an issue.