Whatever the reason for prefixing positive values with a "+", I put together a small example demoing how to do so using DataTable's createdRow option. It is important to note my example does this cell by cell. In the example i refers to the index of each column in a row, so if particular columns need to be ignored add logic to ignore certain values of i. Remember, in JavaScript indexing starts at 0, so your first column is column 0.
library(DT)
sample_data <- mtcars
sample_data[["mpg"]] <- sample_data[["mpg"]] * -1
datatable(
data = sample_data,
options = list(
createdRow = JS(
"function(row, data, index) {",
" data.forEach((d, i) => {",
" row.children[i].innerHTML = (+d && +d > 0) ? `+${ d }` : d;",
" });",
"}"
)
)
)