Sure they can. If they declare war and officially enter the conflict. That wouldn't be a NATO action though, it's a defensive pact.

And it won't happen. Although... the last few days have shown extraordinary events that I never expected. The EU approving 500 Million for weapon purchases for Ukraine, countries supplying fighters after two days delay, officiaks saying that their are fine with their citizens volunteering to fight for another state army (illegal pretty much everywhere), plans to supply troop movement intelligence... That's all just a very small step from active participation.

This is the most significant shift in European politics since the fall of the Soviet union.