To answer your question, the answer is yes, BUT, probably not fully supported. So, I had seen people in the past use interoperable device object for CP appliance, but as @Danny indicated, by definition, 3rd party is non-CP appliance. Now, technically, if its CP device managed by another mgmt server, then you create externally managed object and that is a way to do it. Would that fix your problem? I have no idea, but its right way of doing it. If that fails, then you can engage TAC to troubleshoot further or run below debugs to check:
vpn debug trunc
vpn debug ikeon
generate traffic
vpn debug ikeoff
fw ctl debug 0
fw ctl debug -x
Check vpnd.elg files, as well as ike.elg in $FWDIR/log directory.
Hope that helps.
Andy