No, but you can set up addition keywords with keyboard shortcuts and then quickly tag batches with quick keyboard activations. In fact I have one that I use to REMOVE the "RAW" tag iPhoto automatically adds. So first thing after I import a bunch of shots, they show up in the recently imported window all of them already selected. Command-K opens up the keyword panel. And for me a shift-X (capital X) un-tags all the photos of the "RAW" tag.
Likewise I tend to tag with other common tags like which lens was used for the shots, so a quick jab at the "2" key tags them all as "12-60mm", a 5 gives me my 50-200mm lens tag, etc. Likewise for you, a press of "i" would tag them all as iPhone.
Nice thing is you don't even need to take your hands off the keyboard. After import since they are all preselected, command-K gets the keyword panel and your shortcuts puts in all the default values you want to apply first to the entire batch.
Patrick