it is possible, but i don't believe its possible with a huge pokemon like steelix.
you see there's a program which allows you to change the over-world sprites that is called poketex. using this program i will now demonstrate how you can bring your idea into life.. or hgss ...
Each sprite has a limit to it's size.. let me show you see the red square around pidgeot? thats the limit.
but for steelix its different so its harder to work with, to be honest i have no idea how to turn this huge sprite into a bike. but what you could do is, use another pokemon for a bike. each of the over-world sprites has been inserted in the game as number picture thingies...ahh a picture is worth more than a thousand words... you could combine the numbers by copy and pasting them, however you may have some problems with the color palette.
hope the information helped