You'd have to use "{EXP:}" for that.
Try
Set decimal [dec1] value to 50,00000
Set decimal [dec2] value to 25,00000
Set decimal [dec3] value to the converted value of {EXP: {DEC:dec1} + {DEC:dec2}}
Write '[Purple] {DEC:dec3} = {EXP: {DEC:dec1} + {DEC:dec2}}' to log
Put the token in the "Convert Text/Token" field.
If it writes "Not Set = Expression error" to the log, let me know whether you use a comma as the decimal separator.