library(dplyr)
bind_rows(df2, df1[1] %>%
                 transmute(ColumnC = as.character(ColumnA)))
#  ColumnC ColumnD
#1       a       b
#2       1    <NA>
#3       2    <NA>
#4       3    <NA>

df1 <- data.frame(ColumnA = 1:3, ColumnB = 4:6)
df2 <- data.frame(ColumnC = 'a', ColumnD = 'b',
         stringsAsFactors = FALSE)