How to create subcategories for each row in a data table in R

I am trying to create a data table that sorts each of the variable rows into 2 different subgroups. Here is a reproducible example to hopefully clarify what I'd like to do.

table <- mtcars %>% 
  group_by(vs) %>% 
  summarise(MPG=mean(mpg),DRAT=mean(drat))

table=t(table)
table

          [,1]      [,2]
vs    0.000000  1.000000
MPG  16.616667 24.557143
DRAT  3.392222  3.859286 

#here is what I would like the output to look like, but am unsure how to do create it (some of these numbers I just made up) 

                    **[,1]      [,2]
vs               0.000000  1.000000
MPG (overall)   16.616667 24.557143**
   MPG (am=1)         14        13 
   MPG (am=0)         11        12 
**DRAT (overall) 3.392222  3.859286**  
   DRAT (am=1)        3.1       3.6 
   DRAT (am=0)         4          4 

So with that table in mind, I want to modify it so for the MPG and DRAT variables, there are two subcategories based on whether cars are in the 0 or 1 group for the am variable in the data set. I'm having a hard time telling R to have column groups for the vs variable, and also separately analyzing the am groups (0 or 1) for each row and displaying that in the table. Thank you!