Ok, so to paint on an imported texture in ZBrush you first have to apply it to the model as Polypaint - otherwise it won't work. So load your texture. go to polypaint under tools and choose Polypaint from Texture.  Then make sure the texture map is not active anymore and start your painting. When you're done, you'll need to go to Texture Map under the Tools menu and choose Create>New from Polypaint.  Note that if you want sizes larger than 2048 you'll need to go to UVMap under Tools and set the size and UVMap border you want. I usually have a size of 4096 with a border of about 8 pixels.