These codes worked for me:
$BEXP Part 1
043CAADC 48694000
$BEXP Part 2
043CAAE0 48694000
$BEXP Part 3(Ike)
043CAAE4 48694000
They're just the default ones from geckocodes.org https://geckocodes.org/index.php?c=RFEE01
Also, as for giving units skills, the codes on that page do work, but only for normal mode.
However, to add skills that take capacity, you need to give a value to that capacity or it won't show up. Add 4 bytes to the address, so for example to give Micaiah Vantage in her 3rd skill slot, you need
04886DF4 807101B0
04886DF8 00020000
This makes it use 0 capacity but it does show up.
Put 00030000 instead to give it a lock as well.
If you put any other value (haven't tested much but I'll assume as much) it'll cost the default capacity, which is 10 in the case of Vantage. Sadly you can't make it cost something else, only default or 0, but there are tools to manually edit these into the roms anyway).
Sadly I don't know of codes for easy or hard mode... someone else might, though. But at least the BEXP codes work on all difficulties.