That didnt quite make sense to me, however.
The answer is Yes, and No.
If you cancel the download, then No you cannot resume it.
If the download fails, through no fault of your own, mIRC can attempt to resume it, if you goto, ALT + O - DCC - 'If file exists' You will see an option there in a dropdown box, 'Resume', though it might be worth mentioning, you should NOT rely on this method of be successful at all times as it is not guarrenteed to resume.
Eamonn.