How do you add suffix to the word populate?

You can add various suffixes to the word "populate" depending on the meaning you want to convey. Here are some examples:

-ation: Population (the number of people living in a place)

-ed: Populated (filled with people)

-ing: Populating (the act of filling a place with people)

-er: Populator (a person who fills a place with people)

-or: Populator (a person who fills a place with people)

-ly: Populately (in a manner that fills a place with people)

The specific suffix you use will depend on the specific context and the intended meaning.