Two main choices
1) You overwrite existing characters with the ones you want. Works the same as overwriting any characters (hardest part being if they are different tile sizes and you have to figure out that format as well). For characters with accents in European languages it often works better using the Japanese ROMs as a base by virtue of them usually having loads of characters to overwrite where English translations might drop the characters down to A-Z,a-z,1-9, and .,?! or something not much better -- sometimes if your chosen language only uses some Roman characters for loan words or are incredibly rare (and you can alter the script to not need them) then you can possibly get a few more characters that way.
2) You expand the encoding to include extra space for those characters. Depending upon the system and format of a given game this can be quite easy (the DS' NTFR being an example here) or incredibly difficult (most other things where you have limited tile space in the VRAM and ROM, which you also have to tell the game to use, being examples here, megaman likely being in this family).
Some split the difference and overwrite and change encoding so those extra characters are more in line with more conventional encodings but that is all you.
I don't think I have ever seen anybody actually add accents as a third pass for anything other than video subtitles so I will skip that one.
As far as specifics for megaman 7 then as it is a mainly megaman game I would not be surprised to see either the wiki here or one of the megaman/rockman fan sites/ROM hacking sites have such info.