Periwinkle doesn't need much except some well drained soil and a little fertilizer. You can use the miracle grow to get the current plants bigger, but they spread mostly by seeds. So just get them to bloom and the rest will happen.
If you really want to spread them faster, you can get more plants or take cuttings and plant them closer together. I still would not plant them closer than 18 inches because each plant can bush out that far.
I think you really only need to have some patience.
Mulch between the plants to keep the weeds down.
Happy gardening in Hawaii. Gardens are where people grow.