Is it possible to add an LTE modem into a CCR1009-7G-1C-1S+ via the SmartCard slot, Mico SD slot or USB connector?
Yes, via USB.
Check this page out to get an idea of what may be supported: https://help.mikrotik.com/docs/display/ROS/Peripherals
There are Mikrotik LTE devices as well…
Not USB though…