Yes, you can add a new creative to an Advantage+ sales campaign during the learning phase—it’s designed to handle multiple creatives. However, adding new assets may restart or prolong the learning phase slightly.
Better Approach (If You Want to Minimize Disruption):
Duplicate the campaign or ad group, and add the new creative there.
→ This lets you test the new asset without affecting current learning progress.
If you're still early in the learning phase (under 50 conversions), it's fine to add now. Just avoid frequent changes once optimization kicks in.