Prefix and suffix are verbs. Using append when one means suffix creates ambiguity because verb append has the longstanding meaning "attach, affix" which could be done by prefixing or suffixing. I'd recommend using prefix and suffix when that's what you mean (and append when you don't wish to specify whether something is to be attached as a prefix or suffix).
Adding insult to the injury, prepend has apparently become a widely used technical-jargon de facto synonym for prefix. It's a newly coined word meant to mean the opposite of the mistaken understanding of append that I just mentioned. It's unnecessary and perpetuates the misunderstanding. Just use prefix.
Transitive verbs: (source Merriam-Webster 2001)
- Append: 1. attach, fix 2. to add as a supplement or appendix (as in a book)
- Prefix: 1. to fix or appoint beforehand 2. to place in front; especially : to add as a prefix
- Suffix: an affix occurring at the end of a word, base, or phrase -- compare PREFIX