Can I add units from base with an ambush?

No — Ambush doesn't touch units sitting at your Base. It's only a permission for playing a unit card from your hand to a battlefield where you already control units, plus [Reaction] timing while you do that (Rule 822.1.b). It never moves a unit that's already on the board.

To get a unit from your Base onto a battlefield you use its Standard Move: exhaust it during your Main Phase in an Open State, and not during a Showdown or Combat (Rule 144.1). Ambush changes neither the cost nor the timing of that.

If you actually meant playing an Ambush unit to your Base, the Reaction half doesn't apply there — Base is [Action] timing only (and playing to Base needs no special permission anyway). Ambush is what lets you drop a new unit onto a battlefield mid-fight, not what gets your Base units into the fight.