how do you insert town maps?
Search the turoials section for a world map inserting tut by haefele.
As far as I know you can put them anywhere.
Make a new person in pallet town, give them a script, open it up in XSE, make them say something and add in a giveitem. Check RaiRai-Kun's XSE Tut for item numbers in hex.
geting a cubone lv 15 with pokerus,
givepokemon 0xcubone 0xE 0x0 0x0 0x0 0x0
You can search around for PokeRus, but as far as I know I don't think anyone knows how to do it.
I don't think there is a behavior byte for it in third generation games.
givepokemon 0xeevee 0x7 0x0 0x0 0x0 0x0
giveitem 0xpokeballs 0x5 0x - 0 if it's a person - 1 if you're finding them on the ground
I don't know the item or pokemon numbers offhand or even in hex, so just hit up RaiRai-Kun's tutorial for those.