DC-DC converter maximum current rating
There is an option to add up to 470uF on the converter's output and this will alleviate the situation in that it spreads the pulse of current you are taking over a longer time period with a smaller peak seen by the converter.

The problem is that the manufacturer states that 470uF is the max you can add and I suspect that if you added more it might prevent the converter working correctly. So this is to be avoided because, it probably needs to see a controlled amount of ripple on it's output to function properly.

A way to get round this is to use an inductor in series with the output then a decent sized capacitor to ground probably in the region of 1000uF. The inductor value depends on how long your surge lasts. The inductor "buffers" the motor output current peaks from the converter allowing it to still produce the ripple it needs to operate correctly.

The converter specifies that it's output ripple is 150mVp-p and the inductor is there to allow this to largely remain at that level. If I was at my work desk I'd simulate the effects but I'm not