Reference another dataset in a calculated column without combining them

Hi everyone,

I have a dataset like this: 

KEY Threshold
1 2.1
2 2.3
3 2.5

Then I have another dataset with lots of entries, and I need a calculated column that takes the filtered KEY(only one at a time) in order to retrieve the threshold and calculate the final value.

I know that there is a possibility for this, but it would triplicating the entries in my other dataframe so that there will be one entry for each KEY, but this would make my dataset huge.

Is there a way to get the threshold from the other dataframe without combining them?

If not, would it be possible to create variables or constants that change when a KEY is filtered?

Thank you very much.