If the board did not come with a center fin, I would not modify it to take one. You have 2 options as far as I know.
1) Learn to use your edge and better control the board. Many here take off their center fin, giving them a more slippery feel, but since they have learned to use the edging characteristics of the board correctly, they really do not need the center fin. Some refer to the center fin as a "crutch" or similar to training wheels on a bike. The board should edge fine. You just need to maybe get a better feel of the edging.
2) Get another board that either you like with or without a center fin, or one w/o a fin that maybe has better edging feel than the one you own now.
I am not familiar with your board, but like I said, if it was not made with a center fin, I would not modify it.
Whew! Too much coffee today! Sorry!