As seen in the pronto FAQ on this site, not been able to try it with my ipronto, but looks like a "dirty" and good way of getting it to work......
B8-03 How can I get the Pronto to "hold" a button for several seconds in a macro? Basically, there is no particularly easy way. However, a workaround is available that involves splitting the signal up (thanks to Eric Johnson):
Learn the desired button by briefly tapping it on the original remote. Do not hold down the button as is generally recommended.
Learn the same signal a second time – but this time start pressing the button on your original remote before pointing it at the Pronto/RC5000. Then, while continuing to hold the button, have the Pronto to learn the signal and wait until it is finished. I recommend the "duck blind" method for this. That way, the Pronto will learn the repeat portion of the signal, but not the preamble.
Create a macro that looks like this: [C] Signal Beginning [C] Signal Repeat [C] Signal Repeat [C] Signal Repeat [xxx] [C] Signal Repeat
Add the signal repeat command as many time as is required to reach the desired transmission time. Quite a few repeats may be required.