Rashi on Numbers 32:14/רש"י על במדבר ל״ב:י״ד

1לספות is similar in meaning to the verbs in (Isaiah 29:1): “add (ספו) year to year”; ( 7:21): “add (ספו) your burnt-offerings” — an expression denoting adding (The translation is: to add even more to the Lord’s fierce anger against Israel).