Hey thestig,
Yes, you can add lines up to about 40m in total length, but no longer. It will increase the length of the power stroke for a more sustained power dive which will probably give you what you need.
Did you know that you can also creat power on the "up stroke". By sheeting out a little, the angle of attack of the foil to wind direction will be eased and will allow the kite to increase speed and subsequently increase apparent wind power. Another tight turn and the second dive, and you'll be up and away.
Now for the negetive: By adding line length, you also add lines at perpendicular angle to the wind, which equals a fair bit of drag. Drag from the lines, at the kite end, will hold the kite back in the wind window ie, you'll lose upwind ability which will make it harder to stay upwind especially in marginal winds.
The next problem is that the back lines will droop with the line drag and hang in big loops back in the wind. This adds slack to the steering lines and will make your steering less responsive although it should be OK with 40m, to go longer becomes very hard to steer and go upwind.
These problems can be offset to a degree by using thinner lines, but that just starts getting silly.
Buy yourself a set of 10m line extensions and have fun! It's all good! Other option is I'm sure you could find a second hand or even superceded bow kite for a few hundred dollars and voila!
Good winds,