Whether it's a simple job depends on whether you have access to the ceiling space above the bathroom. If so...
Turn off the breaker or remove the fuse. Open up the box enclosing the pull-cord switch, remove it, and in its place attach one end of a length of romex cable (or equivalent) with wire nuts. (Attach the new cable's ground connection too, of course.) Route the other end of the romex out of the box, through the attic and across to the wall where you want the switch, then down through a hole bored in the header, and out a hole in the plaster adjacent to a stud, where you want the new switch. Slip a switch box over the romex an d secure it to the stud. Install a switch there which makes/breaks the connection between the romex's black and white wires, connect the safety ground wire, mount the switch into the box and test.
Recheck your work. In particular, please make sure the wiring colors match up properly, to help protect the next person to work on this circuit.
Close the boxes, plaster, prime, paint.
(Note: I have no idea whether this will meet code requirements. The US doesn't generally have either electric showers or pull-cord switches. In fact, modern US construction puts bathroom light switches inside the bathroom, just as with any other room; the New England area is notably odd in having many houses which use the old switch-outside-the-bathroom arrangement.)