Hi,
If it is char, it will work.
data lv_charg type mchb-charg.
lv_charg = '1234'.
CALL FUNCTION 'CONVERSION_EXIT_ALPHA_INPUT'
EXPORTING
INPUT = lv_charg
IMPORTING
OUTPUT = lv_charg
.
write lv_charg.
If not, check what is happening in debugging and find what is getting passed as input and what is coming as output.