Either expand or offset would work for your needs, IMO offset would be more precise initially. There may well be some old software for editing the MEP4, but be aware the MEP4 went out of production many years ago (early '90s if I remember correctly). There were a few similar devices made (some much later time frame), I started using the MEP4 in '87. Once you get used to hexadecimal numbering it gets much easier and really doesn't require that much time to program (5-10 min for each of my user memories I make). And there are likely some computer programs that could do the same thing, but I can't say that for sure. I have never searched for those, or have I looked for a computer editor, I took the computer out of my system many years ago because I prefer to work with hardware rather than spending time solving software, connection, and other assorted issues rather than playing or composing. And when you find that piece of software that is easy to use and does exactly what you want, the computer manufacturers will change their OS and many times that software will no longer run when you get your new computer and the software manufacturer may or may not upgrade the software for a new operating system. For many people, computers work well integrated into their keyboard system, I just got tired of the headaches and constantly having to buy new software, the learning curve with each new piece of software, and the various other assorted computer issues that seemed to plague my system. So decide what choices work best for you and go for it, you can reach the same end result from many different roads/paths, do what you feel comfortable with. If you do get an MEP4, I'll be happy to answer any questions about it that I can.
DX7IIFD, SY77, SY99, Hammond C3, Steinway L, CP300, AW1600, etc.