Now, for actually inserting the tile.
What you are going to have to do is open up the tileset (after saving is as a DIB, of course). Then, expand it slightly. You will be using this area for palette converting. Afterwards, compile a palette of all of the colors in the original tiles. Paste the tiles you want to insert into the newly created open space. Next, take the palette that you compiled and use the Eye Dropper and Paint Bucket tools and "replace" the colors of the desired tiles to the ones of the palette of the tileset. Now, allign the tiles so they fit in the original tileset area (you might need to replace some existing tiles), and then resize the image so that it is back to its original size. Save, then import in A-Map.
Hope this helps! :)