There are lots of possibilities, e.g.
library(stringi)
p1 <- stri_sub("3990", from = 1, to = 2)
p2 <- stri_sub("3990", from = 3, to = 4)
paste0(p1, ".", p2)
You can use that approach and write a function.
Another solution (without stringi package)
x <- "23232323532232"
whereDot <- 5
splitX <- substring(x, c(1, whereDot), c(whereDot - 1, nchar(x)))
paste0(splitX[1], ".", splitX[2])
"2323.2323532232"