Thinking it through, the 555 emits a sink or source pulse on pin 3 at regular intervals as determined by the resistor-capacitor setup on pins 2, 6 and 7. The 4017 simply responds to this pulse by advancing one count until it reaches 10, then starts over. To gradually slow the count, you would have to systematically alter the duty cycle of the 555, lengthening the off space each pulse until it counts ten, then repeating for each cycle.
I can think of no way to do this with just a 555 and 4017, although some of the wiser may.
It could be easily done with a PIC, however. (OMG! I'm beginning to sound like THEM!