It's possible right now.
You need 3 tools, DB Master 11, FIFA 11 Texture Editor, Mini Kit Tool.
First open FIFA 11 Texture Editor and "data0.big", find "fifa_ng_db.db" and click on "Edit" then "Active .big file's into folder". Repeat for "fifa_ng_db-meta.xml".
Open DB Master 11 open "teamkits" table and in the "teamtechid" find the id of the team you want to add third kit (or gk the procedure is the same). To get the id of the team go to the "teams" table and search it by "teamname", the id is in the "teamid" column.
Back on "teamkits" after you find the team, select the row with "0" in the "teamkittypetechid" column (or the 1, 0 is the home kit, 1 the away kit, 2 the gk kit, and 3 the third kit, I suggest you select the one that looks the most to the third or gk you are adding), go to "Record" and "Copy", then in "Record", "Insert", now we have a duplicated kit, it will be at the bottom of the table, go find it and change "teamkittypetechid" with the id you want.
Back to FIFA 11 Texture Editor, go to "Edit" "Search file" and search for "kit_TEAMID_" without quotes. On the first find click on "Edit" and then "Active .big file's into folder". go to your "Game" folder, then "data", "sceneassets", "kit", find your recent export, duplicate it and change the name to "kit_TEAMID_2or3_0.rx3" (example "kit_1377_3_0.rx3"). Open that file in FIFA 11 Texture Editor and edit it.
Now for the mini kit, go to FIFA 11 Texture Editor, and open data1.big, search for "j0_TEAMID" or if the id is longer than 4 characters "j0_0TEAMID" (must be at least 5 characters long, example "j0_00010", "j0_01377", "j0_10030" or "j0_111377") and make it active in folder. Go to the "Game" folder, "data", "ui", "artAssets", "kits" and rename (duplicate if you will edit the home mini) the recently exported file and change "j0" to "j3". Go to Mini Kit Tool, find the team in the list and select third, import the texture, and click on make. Test in game!