Help with tilesets

Yeah im kinda new to this and ive been messing around with the RPG Maker XP. And when im making a map i choose the tilset for it, but say it dosent have something in it that i need, like a door, ill try and change the tileset to something which dose have a door but then everything on the map will change to random things on that new tileset i put up. I was wondering how i could solve this problem? 

Open MS Paint and add to the original tileset you used from the new one.

Is there no other way?

in rm2k/3 they let you change chipsets,maybe you can do that in XP if not then that is the only thing i can think of

I'm certain it would be more helpful to the subject if you elaborated further, mister nox.