Of course you can use your riding car.
You will want to put another "hook" on the 5" riding car, offset from the centre so that the coupling bar between locomotive and riding car is straight.
I do have a 4-3/4" gauge riding car that I use when at tracks with 4-3/4" gauge (almost all over here in North America that cater to the smaller gauges; 5" is not popular at all, my local club excepted).
Another JohnS