Frostbite damage - does damage over time add or multiply with spell damage?

I have a frostbite build and am trying to optimize my damage. Im hitting 999 stacks on bosses if i can free casts for 5 seconds so im looking for other ways to increase damage.

I already have Snowdrifts and am stacking freeze multiplier for the cold pen multiplication.

The only other thing I can find to increase my damage is possibly damage over time. But as far as I understand, there’s no reason to get that over cold/spell/elemental damage because the damage over time is additive, not multiplicative.

Is this true, and if so why would you ever prioritze damage ober time over more feneric increased damage sources?