4 mags up front is plenty for me. I normally have a double BFG Ten Speed pouch on the front of the Spiritus, and two more on my PC cummerbund, plus a shooters belt. I could comfortably carry 9-10 MP5 mags if I really wanted to...
Do you mean quick-detatch (QD)? If so, it's already QD - the velcro panel is the only part that's threaded onto the plate carrier. I just either swap inserts into the Spiritus rig, or I use a different placard (Placard example: https://www.uktactical.com/product/warrior-assault-systems-warrior-removable-triple-molle-open-pouch-for-rpc-multicam-12395)
If you're talking about mounting specific pouches on there instead (say you wanted two 3x sub gun pouches), you can get blank placards too: https://www.uktactical.com/product/warrior-assault-systems-warrior-molle-front-panel-for-recon-plate-carrier-multicam-12028
The T Bar buckles are at the British Tactical link I posted originally under "2 x Female Molle T-Bar Buckles"