Can I summon units from my hand to a battlefield I'm currently attacking?

Ruling: No, you cannot summon units to join an attack at a battlefield you don't control.

Sequence:

• Moving a unit to a battlefield creates a showdown
• During a showdown, base speed cards (including most units) cannot be played
• Reaction speed units exist but require you to control the location where you're playing them
• As the attacker, you do not control the battlefield

Nuances:

• This restriction applies even to reaction speed units like Shen, because you must control the battlefield to play units there
• If you control a battlefield and charm an enemy unit there to start a showdown, your opponent cannot play reaction units like Shen because they don't control that battlefield