If you mean normal variables in a program, then it is of course possible. If you are thinking of the variables implemented internally for e-mailing (parameter: MSGVARS), then it is not possible, but you should be able to build up the logics (and calculate if needed) in the program executing the MRGSPLFPDF command.
Re: Using variables in the new fields in the MRGSPLFPDF command
If you mean normal variables in a program, then it is of course possible. If you are thinking of the variables implemented internally for e-mailing (parameter: MSGVARS), then it is not possible, but you should be able to build up the logics (and calculate if needed) in the program executing the MRGSPLFPDF command.