Yes you can, I use 85 watt CFL which is the same as a 450 watt standard, you must take in consideration of the extra heat, and adjust distance for umbrella use so as not to create a fire hazard. Cowboy studio has a standard medium based socket, so as long as your bulb has a medium base it will work.
rlionheart is correct. However, I would recommend that you add an additional piece to your kit as I have done and stop at any local hardward store to purchase a light reflector Google "Optical Light Reflector." This will help with your light control when using a higher wattage bulb. It will also reflect the heat produced from the bulb away from your subject and allow for greater control of your light.
rlionheart is correct, for safety reasons they ask that you not use stronger bulbs, but I do as well, since a 45 w does not provide much light. But be more cautious of the surroundings when setting up and always check your lamp socket for heat damage before use.