You cannot since it is hardcoded into the ability. You need to simulate the Harvest ability to do that.
Here is my incomplete demo map of such a system.
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Considered easy altering of the unit textures?
http://static.sc2mapster.com/content/attachments/15/252/Screenshot2011-04-17_09_16_21.jpg
For my public assets look here!
http://www.sc2mapster.com/forums/resources/data-assets/46733-the-assets-of-evil/#1

Gwypaas Regular ShmoeHello, I was able to allow several units to harvest from one mineral field using the "Bypass Resource Queue" flag. The problem now is that when they are harvesting they have no collision size create which creates huge mess of units instead of spreading them out and waiting for their turn due to collision. How would it be possible to turn on collision for them?