Sure it can. 

Oh, but what code and/or quality do you want in the final product?

He Is In Control, Have a Great Day,  Brent

Yes, bare galvanized sheet metal is resistance welded every day in many industries.

The best process is one that first melts the galvanized coating which then squeezes out from beneath the copper electrodes before the weld nugget forms at the faying surface between the steel sheets.

The major caution to be aware of is that sometimes if the amperage is too low, or the force is too high, just the galvanized coating might melt and stick with no weld nugget formed.  However, this is rare, and is  easily avoided by doing peel tests during process qualification to verify that sound weld nuggets are formed. Galvanized coatings also might increase the wear rate for some electrodes, resulting in shorter maintenance intervals, and the workaround for this realized by establishing well documented maintenance intervals for each individual process setting.

Clean surfaces also contribute to quality, so best practices might not allow welding over grease or dirty surfaces - and this could be verified by testing. The carbon content in the oil MIGHT cause some undesirable hardness (and brittleness?)  increases in SOME steels - if the oil increases the carbon content in the weld; however, in most cases, the steel is clean - and the oil might even be burned up or expelled before the weld forms.

I hope that response was short enough, without missing the most important details.

The Resistance Welding Handbook (RWMA) used to have process settings for many different conditions.