Make column titles span over subgroups

Question

Consider the following MWE:

library(expss)
library(openxlsx)
data(mtcars)
mtcars = apply_labels(mtcars,
                      mpg = "Miles/(US) gallon",
                      cyl = "Number of cylinders",
                      disp = "Displacement (cu.in.)",
                      hp = "Gross horsepower",
                      drat = "Rear axle ratio",
                      wt = "Weight (1000 lbs)",
                      qsec = "1/4 mile time",
                      vs = "Engine",
                      vs = c("V-engine" = 0,
                             "Straight engine" = 1),
                      am = "Transmission",
                      am = c("Automatic" = 0,
                             "Manual"=1),
                      gear = "Number of forward gears",
                      carb = "Number of carburetors"
) 

tbl1 <- mtcars %>% 
  calc_cro_cases(cell_vars = list(hp),
                 col_vars  = list(total(), am))

# save as spreadsheet
wb <- createWorkbook()
sh <- addWorksheet(wb, "Table 1")
xl_write(wb = wb, sheet =  sh, obj = tbl1)
saveWorkbook(wb, "table2.xlsx")

The exported spreadsheet looks like

The title "Transmission" does not span over both subgroups. There is at least no border to the right of the title but if I want to center the header, it would be nice to make it span over both subgroups.

Is there a way in expss to make that happen?