While putting on equipment usable by your subjob is written to be possible server-side, there are still client limitations that will prevent you from doing so. You can use the "!flip" command in-game to work around this. This command will send a packet to your client's UI to visually flip your main and sub jobs. This UI packet is purely visual and does not actually change your jobs. When you are done equipping, use "!flip" to toggle this feature off.
This command can be used in conjunction with the "!dw" command to equip a sub weapon usable only by your subjob. More info on that command is here.