With respect, FreePBX is very good at what it does, this use case is to my current knowledge not covered as yet, if you find a way to that within the GUI, then please post it.
I have had to do a similar thing a long time ago, to maintain continuity (and discontinuity ) of mixmonitor over various dialplan transitions. I posted my analysis of what needs to be done and why, and my particular approach to the problem’s resolution. in my case the agant(s) just need toggle *9 while in a call. the agi script will do all the heavy lifting, including stating that the recording is paused/unpaused with a time stamp and a reason when that is needed. Asterisk is in itself a lot more than FreePBX 
JM2CWAE