I've never used this device but had a look at the manual. On page 23 and 24 it shows where you put in the settings for your VoIP provider. This is where you would change the settings so that the MV-372 becomes two extensions off of the 3CX. You could then add the two VoIP lines from your provider as two trunks going into the 3CX. That would give you access to those trunks from any extension, including the two GSM lines, off of the 3Cx board. You will have to change around/create the dialling plans in both the 372 and the 3CX.
If you make changes, just make sure you record what the original settings were in case you have to go back. There may be a number of things you will have to "tweak" before it works the way you want it to.