With the R152/153 (also other OpenBve trains with the exception of a few) trains, I'm pretty sure you have to access the Exterior folder and edit the signs manually.
The R38 and R175 however have rollsign commands.
R38 commands
8=Top Rollsign
9=Middle Rollsign
0=Bottom Rollsign
R175 commands
4=Front Bullets
6=Side Signs