1. Combining several relays in parallel is not good, because the contacts do not switch at exactly the same time, and it will cause arcing at the faster one. Another thing is that the motor is an inductive load and needs more current than the rated \$15\,\mathrm{A}\$ during startup.
2. Without modifying the existing circuit, it can be done with another relay with 12V coil and contacts rated for inrush current of the motor.

1. Combining several relays in parallel is not good, because the contacts do not switch at exactly the same time, and it will cause arcing at the faster one. Another thing is that the motor is an inductive load and needs more current than the rated \$15\,\mathrm{A}\$ during startup.
2. Without modifying the existing circuit, it can be done with another relay with 12V coil and contacts rated for inrush current of the motor.

Edit: Two 10A relays in series can still carry only 10A. Theoretically it should be possible to double the current by merging more relays to operate like one larger relay, but is not possible as described above.

Possible wiring can be done like this, for example with 30A or 40A automotive relays: