it's possible but complicated to do
for example the name table start at offset 1F7177 (ruby)
you have to declare all the new names,, after you should first of all change in routine for copy all the names ^^"
You should also declare the moves they use, the place, the footprints,...
and above all put the new sprite...
There is a patch of jambo51 to fire red for increse pokemon to 649 :)