Possible to add an inkjet head to the Shapeoko?

Is it possible to add an inket print head to the Shapeoko to make a flatbed injet printer?

Yes, but you’d need a different/separate controller for the inkjet mechanism.

What do you want to achieve? There are already devices which do this, e.g., for customizing cakes:

https://www.ink4cakes.com/

The reprap folks have done some work on paste extruders, and we should always remember that the principle of inkjet printing was discovered by accident when a soldering iron was touched to a syringe which for some reason was filled w/ ink.

The objective is to print on large non paper objects and not spend a fortune on commercially available flatbed inkjet/UV printers.

Single color (black) is fine for my primary purpose, but would like to eventually do color.

The X-Y-Z mechanism is already in place, looking for an attachment to print, plotting with a pen probably won’t work very well due to the volume I am anticipating.

How fine a line do you need to lay down, on what material using what sort of ink / paint?

There was at least one instance of an airbrush being mounted:

.3 mm would be fine enough to start out for immediate needs. I’m sure once I get into color I would require finer lines.

My inclination would be to start w/ an airbrush — just use a canister of compressed air to start, then if it pans out you’ll just need a small compressor.

You’ll want to wire up a mechanism of some sort to actuate the air brush (alternately, you might want to consider having it always spray and instead using a sponge shield) which is controlled by the toolchange / spindle start / speed command.