This can definitely be done, but it requires some of the advanced features of smartthings. It certainly not basic. 
I’m not feeling very well today so I won’t be able to walk you through all of the details, but I will give you the top level approach and then other people can help with the specifics.
Smartthings allows you to create “virtual“ devices. These exist only as code, but smartthings and Echo will treat them just like any other physical device.
So in your case, you will create a virtual motion sensor. Then you would turn that virtual motion sensor on based on all of the other conditions that you want to factor in, which could include time of day, obviously activity on the real physical motion sensor, etc.
Even better, you can create a virtual motion sensor that also has “capability.switch” which means that smartthings will see it as both a switch and a motion sensor. So you will be able to turn it on using any of the standard automations, Which will then look like a motion sensor coming on, Which you can then use to trigger your echo routine.
I hope that’s all clear. The real physical motion sensor might be detecting activity all day long, but you could set up a rule so that the Virtua motion sensor only turns on during a certain time range, when certain people are home, and when the real physical motion sensor detects activity. Or whatever set of restrictions you want to reply.
The following FAQ discusses all of this and has the DTH you will need for a sensor combined with a switch. ( I don’t remember if that one is a contact sensor or a motion sensor, but it doesn’t actually matter as the echo routine will trigger off of either and it doesn’t have to be perfect match to the physical device. )