You can use the tidyr::complete
function:
complete(df, distance, years = full_seq(years, period = 1), fill = list(area = 0))
# A tibble: 14 x 3
distance years area
<fct> <dbl> <dbl>
1 100 1. 40.
2 100 2. 0.
3 100 3. 0.
4 100 4. 0.
5 100 5. 50.
6 100 6. 60.
7 100 7. 0.
8 NPR 1. 0.
9 NPR 2. 0.
10 NPR 3. 10.
11 NPR 4. 20.
12 NPR 5. 0.
13 NPR 6. 0.
14 NPR 7. 30.
or slightly shorter:
complete(df, distance, years = 1:7, fill = list(area = 0))