Commerce: Importing Customer Finances - Add Extra Points?

[jennycraftc229]

Hi,

New to M2 Commerce (1 week!) v 2.3.6

I'm trying to give all our customers an extra 500 points for Xmas, but the import feature is overwriting the balance rather than adding to it, does anyone know what the import needs to contain to get the import to add these points?

Tried:

reward_points

500

(result: overwrote points on account)

and 

reward_points

+500

result: n/a, didn't import

Thanks - please save my Christmas Eve!