Yes, CO2 gas concentration can be expressed in units of mg/m³. However, the value is dependent upon temperature and pressure.
At Standard Ambient Temperature and Pressure (SATP) of 25°C and 100 kPa, the conversion is:
1 ppm CO2 = 1.7758 mg/m³
Logger Pro (version 3.13 and newer) and LabQuest App (version 2.72 and newer) include this as a unit change option for the CO2 Gas Sensor. However, note that it is only valid at SATP.