I have never done that on a Yamaha, but I have on many others. Since there are a number of sensors and safety controls involved, you may want to post this question over on http://www.marineengine.com/cgi-bin/discus/discus.cgi They have been a great help to me for a long time. By the way I have found the Yamaha oil injection system to be reliable.
On my 225 HP Evinrude I ran a little extra oil (premixed at 100:1) for about a year before I switched to pre-mix only.
Best regards,
FRank_S