Right then,
To start with if the wheels will move on the axles then yes, just move both wheels in by 3,75mm to give a narrower back to back. If they are plastic wheels moulded as part of the axle your a Turkey at Christmas to start with.
A convenient check gauge is the long side of the Earth pin from a UK 3 pin plug, but you have to get it central. (So as not to unbalance the wagon).
Then again a standard UK mainline wagon will look quite large for OO9 (00 scale on 9mm track} for UK use, some Continental, South African, Australian stuff ran larger loading gauges, on 2 foot gauge but size wise you are nearer to OOn3
(00 on 12mm track) for 3 foot gauge prototypes (eg. Irish Narrow gauge and the Isle of Man, ) than anything that originally ran in the UK
But as the main reason for UK narrow gauge was limited clearances and tight curves, a large piece of rolling stock is a major drawback.
To get more of an idea about 009 and other narrow gauge I suggest joining
(Just click on the above)